What is the point of a GAL?

[socrateaser]

>What court hears the appeal on a custody case?

Court of Appeals (except in NY, where it's called something else, because NY calls its highest court the Court of Appeals).

>Do both parties to the custody case appear to protect their
>own interest or is it just the party appealing as they are the
>ones trying to prove the judge's order in error?

Either or both, depending on who's appealing and whether the appellee wants to defend or not.
>
>While the case has to be argued on law, would the
>unprofessional comments the judge made help in showing his
>judgement was flawed?

The appeal court gets a transcript of the entire record of the case, and you can refer to anything in the record to try to demonstrate an abuse of discretion, bias, prejudice, bad judgment, etc.
>
>
>Also, ss had a GAL....would he still be responsible to present
>ss' interest in the appeal?

Only if the GAL believes that the ss's interests require separate protection, or if the appeals court were to order the GAL to submit a brief and argue.