GPDivx might suit you better (*JUST* possibly) if you don't want to have any sound at all, and don't min about consistency of framerate (at times it runs as fast as 60 fps! Unfortunately, when there's a lot of action it drops to 11). It does have sound if you're willing to use uncompressed PCM at 8kbs ~ 11025KHz. This is not worth it for the space something takes up. Trust me.
Alternatives to GPCinema... well, I suppose there's Moviepark (though that's no longer on sale, and therefore impossible to find unless you owned a GP32 and bought it before it went off the market), and there's also Aquafish's United GP Viewer which claims to open anything encoded for GPCinema. Presumably it may open some other stuff in addition though, or be even more picky - I don't think anyone's really tried it in all seriousness since GPC became freeware.
Oh, and there's GPMP, which I'm pretty sure is a GPCinema demo, though I can't be certain on that. What I do know is that it can't play anything longer than 6 and a half minutes (I did some experiments before I got hold of Moviepark due to lack of a CC to buy the latter with at the time). It also only opens one file called GundapOP.avi in the rot directory, so if you want to use it, you need to set up some kind of filing system and use COMI to move your files around between sections. I did at one stage have a film encoded for it, ant th experience was quite good, but nothing like as good as provided by either Moviepark or GPC.
Other than that... 'fraid not, although I did hear a rumour (and it is a rumour, and one I've not heard anything further about) of someone trying to implement a codec system for the GP32 or something similar rather than having what could be played unique to each player... Personally, I suspect that's either my imagination or something that nothing will ever come from, but hey - it *could* happen. Maybe.
EDIT: Typos...