Gamemode Changer adds an item that lets you easily swap gamemodes. The item looks like a compass and when you right click it opens up a GUI to choose your gamemode.
Select version for changelog:
Changelog
updated it to 1.20.40, removed the message in chat so it will work with the stable scripting :D
mediafire.com/file/e4t9s2tzgywhhmk/Gamemode_Changer_1.20.40_and_above_%25282%2529.mcaddon/file