This is the code for my GCD calculator. I couldn't help myself. It also calculates LCM.

GCD-LCM Calculator - Bugs Fixed -Check It y0

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace GCD_LCM_Calculator
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnGetValue_Click(object sender, EventArgs e)
        {
            if(tbFirstNumber.Text.Contains(".") || tbSecondNumber.Text.Contains("."))
            {
                MessageBox.Show("Key-in only integers.");
                tbFirstNumber.Clear();
                tbSecondNumber.Clear();
                lblValueDisplay.Text = "";
            }

            else
            {
                int firstNumber = int.Parse(tbFirstNumber.Text);
                int secondNumber = int.Parse(tbSecondNumber.Text);
                int absFirstNumber = Math.Abs(firstNumber);
                int absSecondNumber = Math.Abs(secondNumber);

                if (tbFirstNumber.Text == "0" || tbSecondNumber.Text == "0")
                {
                    MessageBox.Show("Values must be non-zero.");
                    tbFirstNumber.Clear();
                    tbSecondNumber.Clear();
                    lblValueDisplay.Text = "";
                }

                else if (absFirstNumber == absSecondNumber)
                {
                    lblValueDisplay.Text = absFirstNumber.ToString();
                }

                else
                {
                    int product = 1;
                    int holder = 0;

                    if (absFirstNumber < absSecondNumber)
                    {
                        var numberHolder = absFirstNumber;
                        absFirstNumber = absSecondNumber;
                        absSecondNumber = numberHolder;
                    }

                    if (btnGetValue.Text == "Get GCD")
                    {
                        while (product > 0)
                        {
                            product = absFirstNumber - absSecondNumber;

                            if (product == absSecondNumber)
                            {
                                holder = product;
                            }

                            if (product > absSecondNumber)
                            {
                                absFirstNumber = product;
                            }
                            else
                            {
                                absFirstNumber = absSecondNumber;
                                absSecondNumber = product;
                            }
                        }

                        lblValueDisplay.Text = holder.ToString();
                    }
                    else if (btnGetValue.Text == "Get LCM")
                    {
                        int nextHolder = absFirstNumber * absSecondNumber;

                        while (product > 0)
                        {
                            product = absFirstNumber - absSecondNumber;

                            if (product == absSecondNumber)
                            {
                                holder = product;
                            }

                            if (product > absSecondNumber)
                            {
                                absFirstNumber = product;
                            }
                            else
                            {
                                absFirstNumber = absSecondNumber;
                                absSecondNumber = product;
                            }
                        }

                        lblValueDisplay.Text = (nextHolder / holder).ToString();
                    }
                }
            }
        }

        private void btnClearAll_Click(object sender, EventArgs e)
        {
            tbFirstNumber.Clear();
            tbSecondNumber.Clear();
            lblValueDisplay.Text = "";
        }

        private void gCdToolStripMenuItem_Click(object sender, EventArgs e)
        {
            btnGetValue.Text = "Get GCD";
            lblValueIs.Text = "GCD is:";
        }

        private void lCMToolStripMenuItem_Click(object sender, EventArgs e)
        {
            btnGetValue.Text = "Get LCM";
            lblValueIs.Text = "LCM is:";
        }
    }
}

GCD-LCM Calculator - Original

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace GCD_LCM_Calculator
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnGetValue_Click(object sender, EventArgs e)
        {
            if(tbFirstNumber.Text.Contains(".") || tbSecondNumber.Text.Contains("."))
            {
                MessageBox.Show("Key-in only integers.");
            }

            int firstNumber = int.Parse(tbFirstNumber.Text);
            int secondNumber = int.Parse(tbSecondNumber.Text);
            int product = 1;
            int holder = 0;

            if (firstNumber < secondNumber)
            {
                var numberHolder = firstNumber;
                firstNumber = secondNumber;
                secondNumber = numberHolder;
            }

            if(btnGetValue.Text == "Get GCD")
            {
                while (product > 0)
                {
                    product = firstNumber - secondNumber;

                    if (product == secondNumber)
                    {
                        holder = product;
                    }

                    if (product > secondNumber)
                    {
                        firstNumber = product;
                    }
                    else
                    {
                        firstNumber = secondNumber;
                        secondNumber = product;
                    }
                }

                lblValueDisplay.Text = holder.ToString();
            }
            else if(btnGetValue.Text == "Get LCM")
            {
                int nextHolder = firstNumber * secondNumber;

                while (product > 0)
                {
                    product = firstNumber - secondNumber;

                    if (product == secondNumber)
                    {
                        holder = product;
                    }

                    if (product > secondNumber)
                    {
                        firstNumber = product;
                    }
                    else
                    {
                        firstNumber = secondNumber;
                        secondNumber = product;
                    }
                }

                lblValueDisplay.Text = (nextHolder / holder).ToString();
            }
        }

        private void btnClearAll_Click(object sender, EventArgs e)
        {
            tbFirstNumber.Clear();
            tbSecondNumber.Clear();
            lblValueDisplay.Text = "";
        }

        private void gCdToolStripMenuItem_Click(object sender, EventArgs e)
        {
            btnGetValue.Text = "Get GCD";
            lblValueIs.Text = "GCD is:";
        }

        private void lCMToolStripMenuItem_Click(object sender, EventArgs e)
        {
            btnGetValue.Text = "Get LCM";
            lblValueIs.Text = "LCM is:";
        }
    }
}