An editor extension makes easier to add a .gitignore file to the root folder of current Unity project

GitignoreGenerator.cs

// Please locate this file as Assets/Editor/GitignoreGenerator.cs
// Then select "Custom" > "Add .gitignore to project root from menu".

using UnityEngine;
using UnityEngine.Networking;
using UnityEditor;
using System.IO;
using System;

public class GitignoreGenerator : EditorWindow {

    static readonly string URL = "https://raw.githubusercontent.com/github/gitignore/master/Unity.gitignore";
    
	[MenuItem("Custom/Add .gitignore to project root")]
	public static void AddGitignore() {
		string projectFolder = Directory.GetCurrentDirectory();
		string gitignoreFile = Path.Combine(projectFolder, ".gitignore");
		if (File.Exists(gitignoreFile)) {
			Debug.Log(".gitignore file is already existing");
			return;
		}

		UnityWebRequest www = UnityWebRequest.Get(URL);
		www.SendWebRequest();

		while (!www.isDone) {
			System.Threading.Thread.Sleep(1);
		}

		if (www.isNetworkError || www.isHttpError) {
			Debug.Log("Error " + www.error);
			return;
		}

		try {
			FileStream fs = File.OpenWrite(gitignoreFile);
			StreamWriter sw = new StreamWriter(fs);
			sw.Write(www.downloadHandler.text);
			sw.Close();
			fs.Close();
			Debug.Log(".gitignore file was created");
		} catch (Exception ex) {
			Debug.Log(ex.Message);
		}
	}

	void OnGUI() {
	}
}