Adding gesture recognizers from code.swift

import UIKit

class ViewController: UIViewController {
    let tapGestureRecognizer: UITapGestureRecognizer! // make a variable to hold the recognizer
    
    @IBOutlet weak var girImage: UIImageView! // add an outlet for your image view
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // make an instance of tap recognizer
        // add your view controller as the target and tell it to call method named "onImageTapped:"
        // Make sure to have that colon at the end!
        tapGestureRecognizer = UITapGestureRecognizer(target: self, action: "onImageTapped:")
        
        // add your recognizer to your image
        girImage.addGestureRecognizer(tapGestureRecognizer)
        
        // enable user interactions or it won't work!
        girImage.userInteractionEnabled = true
    }
    
    func onImageTapped(sender: AnyObject) {
        print("It works from code too!")
    }
}

Swift 3 had been changed:
let avatarGesture = UIGestureRecognizer(target: self, action: #selector(onImageTapped(sender:)))

And the better way is using _ instead using variable in function in this case

 func onImageTapped(_ sender: AnyObject) {
        print("It works from code too!")
    }

Now
let avatarGesture = UIGestureRecognizer(target: self, action: #selector(onImageTapped(_:)))