It’s a reference to players who preform better during a game in comparison to practice perhaps. It can refer to someone who performs better under pressure and when it matters most.

Well game-day means a sports team is going to play and you plan to watch the game on TV. Usually people gather and have food. For example: today is game-day= today X sports team is playing. Guy= man. Here game-day guy means that the person most likely prefers to watch tennis on tv rather than play. This combination of noun-guy/person can be used for many things. For example: I’m more of a burgers and fries girl. Here I’m saying that I prefer burgers( or fast food in general) than eating at some fancy restaurants or trying new foods. You will sometimes hear burgers and fries kinda of girl. Here it’s the same means. Kinda of= type of Hope this helps :)

Game-day in this context would mean that Person B does not like to play sports but rather watch sports. However, if more context was provided this conversation could also mean that Person B only plays tennis best when he is in a competition. Hope that helps!-Kim