Learn Swedish: Mitt drag är ganska dåligt, men spelet är väldigt roligt. - My move is quite bad, but the game is very fun.

No, adverbs never change form to match the noun's gender (en or ett) or number (singular or plural). Even though the adjectives next to them might change, ganska and väldigt will always stay exactly the same.

Why do the adjectives dåligt and roligt end in -t here?

They end in -t because they describe ett words: ett drag (a move) and ett spel (a game). Even though the degree adverbs ganska and väldigt step in between the verb and the adjective, the adjective still has to agree with the noun it describes.

If adverbs don't change form, why does väldigt end in a -t?

In Swedish, you can turn many adjectives into adverbs by adding a -t. Väldigt comes from the adjective väldig (huge or enormous). When you add the -t, it becomes the fixed adverb "very". It will always keep this -t when used as a degree adverb, no matter what noun is in the sentence.

Why does spelet get the definite -et ending, but drag doesn't?