ik ga - jij/zij/hij/het gaat - wij/zij/jullie gaan But if it's a question, it is: ga ik? ga jij? - gaat hij/zij/het? - gaan wij/zij/jullie?
It's the same for most, except for je/jij which changes from 'gaat' to 'ga'..
Why? Maybe someone else can tell you but I can't think of a reason.
If the verb comes before jij it's always conjucated like ik, so nothing irregular there (in most cases this means the -t is dropped, in this case it's -at which is dropped).
The conjugation looks a bit different because the stem ga ends in a vowel, which is a bit exceptional (there are a few more, some examples: slaan, staan, zien). It triggers some standard spelling rules (e.g. ga has only 1 a, but gaat has 2), but for all these verbs jij/zij/hij/het still end in -t and wij/zij/jullie in -n as one would expect.
What's wrong with "Do you attend the gym?" Is there another word for "attend" in Dutch so that "attend" wouldn't be accepted here?