OK theres two main reasons why ogg is better than WMA.
Techncal. ogg files have been shown to sound a lot better than WMA files for a given size.
Licencing. ogg is an open standard so anyone can make a player or encoder without having to pay fees to anyone. WMA on the other hand is a closed standard owned my Microsoft and they will not release the specs unless you pay them a big fat fee, sign hundreds of NDAs and agree to give them a further cut for every copy of the program ship ped (even if you give theprogram away for free). Also the newer versions of WMA support something called DRM, which is a system designed to make music files which will only play one one player. Lots of people think this stinks so they don't use WMA for that reason.
In short there almost no chance of getting a WMA player on the GP32, and even if one was released, you'd be better off avoiding it.
Hope this answers your questions.
Regards,
Baggy