Create a GIF with ImageSharp in MonoGame by storing old frames. MIT license: https://opensource.org/licenses/MIT

GifGame.cs

using System;
using System.IO;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Formats.Gif;
using SixLabors.ImageSharp.PixelFormats;

namespace ImageSharpMg
{
    /// <summary>
    /// Render frames to render targets and create a GIF by pressing 'S'.
    /// </summary>
    public class Game1 : Game
    {
        [STAThread]
        static void Main()
        {
            using (var game = new Game1())
                game.Run();
        }

        private const int FrameCount = 180;
        private const int Width = 320;
        private const int Height = 180;
        private const float Speed = 0.5f;

        private SpriteBatch _spriteBatch;
        private Texture2D _blank;

        private int _currentIndex;
        private Color[][] _frameBuffers;
        private RenderTarget2D _renderTarget;

        private Rgba32[] _rgbaBuffer;

        private const int SquareSize = 32;
        private float _squarePos = -32;

        private KeyboardState _prevKeyboardState;
        private KeyboardState _keyboardState;

        public Game1()
        {
            var graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
            IsMouseVisible = true;
            graphics.PreferredBackBufferWidth = Width;
            graphics.PreferredBackBufferHeight = Height;
        }

        protected override void LoadContent()
        {
            _spriteBatch = new SpriteBatch(GraphicsDevice);

            _blank = new Texture2D(GraphicsDevice, 1, 1);
            _blank.SetData(new[] {Color.White.PackedValue});

            _renderTarget = new RenderTarget2D(GraphicsDevice, Width, Height);

            _frameBuffers = new Color[FrameCount][];

            for (var i = 0; i < FrameCount; i++)
                _frameBuffers[i] = new Color[Width * Height];

            _rgbaBuffer = new Rgba32[Width * Height];
        }

        private void SaveImage()
        {
            using (var image = new Image<Rgba32>(Width, Height))
            {
                var frames = image.Frames;
                for (var i = 1; i <= FrameCount; i++)
                {
                    var frameIndex = (_currentIndex + i) % FrameCount;
                    ConvertColorData(_frameBuffers[frameIndex], _rgbaBuffer);
                    var frame = frames.AddFrame(_rgbaBuffer);
                    frame.MetaData.FrameDelay = 2;
                }

                var encoder = new GifEncoder();
                using (var stream = File.OpenWrite("test.gif"))
                    image.SaveAsGif(stream, encoder);
            }
        }

        private static void ConvertColorData(Color[] mgBuffer, Rgba32[] isBuffer)
        {
            for (var i = 0; i < mgBuffer.Length; i++)
            {
                var c = mgBuffer[i];
                isBuffer[i] = new Rgba32(c.R, c.G, c.B, c.A);
            }
        }

        protected override void Update(GameTime gameTime)
        {
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed ||
                Keyboard.GetState().IsKeyDown(Keys.Escape))
                Exit();

            _prevKeyboardState = _keyboardState;
            _keyboardState = Keyboard.GetState();

            if (_prevKeyboardState.IsKeyUp(Keys.S) && _keyboardState.IsKeyDown(Keys.S))
                SaveImage();

            _squarePos += Speed;
            if (_squarePos > Width)
                _squarePos = -32f;

            base.Update(gameTime);
        }

        protected override void Draw(GameTime gameTime)
        {
            // render the scene to the active render target
            GraphicsDevice.SetRenderTarget(_renderTarget);
            GraphicsDevice.Clear(Color.CornflowerBlue);

            _spriteBatch.Begin();
            _spriteBatch.Draw(_blank, new Rectangle((int) _squarePos, 74, SquareSize, SquareSize), Color.White);
            _spriteBatch.End();

            // Render it to the backbuffer
            GraphicsDevice.SetRenderTarget(null);
            _spriteBatch.Begin();
            _spriteBatch.Draw(_renderTarget, Vector2.Zero, Color.White);
            _spriteBatch.End();

            // store the pixel data
            _renderTarget.GetData(_frameBuffers[_currentIndex]);
            _currentIndex = (_currentIndex + 1) % FrameCount;

            base.Draw(gameTime);
        }
    }
}