May I make a few suggestions? How about “button”, since you also have game pad (controller) and joystick (are you referring to the sticks on a controller or an old-school Atari/flight sim style thing?)? And maybe instead of monitor have screen, just since it seems like a word you might use more often, and can apply to TVs and phones too. I see you’ve added game cartridge – are you more into pre-CD era games, yeah?

I know it’s only *supposed* to be 20 words, but if you’re really passionate about it, I’d say add more if you want! :D Maybe include words you think you might see while playing a game (as these will be useful when you come to actually play a game in Japanese):

Jump, punch, kick, shoot, reload, collect (as in coins or rings), kill, run, walk, dodge, options, menu, restart, win, lose, final battle/round, item, flee, attack, move, turn, backpack/inventory, trainer, capture/catch, route ## (as in Pokemon), objectives, story, mission, multiplayer/singleplayer, RPG, online, opponent, characters, quest, villain/hero, cast (a spell), magic, damage, HP, MP, armour, defense, agility, party (the group you’re fighting with), hit/miss, critical, faint, dead, protagonist, antagonist, NPC, sidekick…

That’s a big (certainly not exhaustive) list that I just typed as it came to me; I’ve no idea where you’d find out how to say all that stuff, but you can maybe just take a handful and ask around :) There are a number there that I thought of with Pokemon in mind ;)