I'm confused by the reaction to は in this exchange
source context: http://neetsha.jp/inside/up/1/2/12840/24.jpg
This is the contrastive 「は」 or at least that is what B takes it to be.
A's line can be interpreted as "You are good at games (if not anything else)!", which is exactly how B interpreted it. That is why B reacts (jokingly) by emphasizing the contrastive 「は」.
B's line is obviously difficult to translate literally as English has no such particles. In that sense, A's line is already difficult to translate to begin with.
Particles can be so powerful that this shortest exchange can stand as a valid joke in itself. (And this often takes place with the contrastive 「は」 in real life.) The 「は」 in B's line would receive much stress in actual pronunciation.
B could have also said:
The normal and neutral way to say "Someone is good at games" is "(先輩は)ゲームが上手い". Using は instead of が in this position is almost certainly taken as "contrastive wa", i.e., it implies he is good only at games and not good at anything else.
Maybe the girl was careless and incidentally leaked her inner feelings, and it hurt 先輩 all the more for her innocence. Or maybe she intentionally said it as a sarcasm. Native speakers of Japanese don't know grammatical terms like "contrastive wa", but they can instantly feel the critical difference between は and が here.
The function of は is twofold:
In this case the important bit is the second function. 「ゲームは上手いですよね」 can be interpreted as, "I don't know about other things, but you sure are good at games!"