Adobe Flash doesn't seem to work in Garry's Mod for Mac OS X anymore, sorry. Your best bet is to. Wasn't the garrys mod slot limit 127 players? 5 years ago from. I'd be interested to hear the results from this. If you can't handle it, why not deny invites after a 'cap' for the small party? Page 1 of 1 1.
The only thing I can suggest since it seems the video is the issue (other than using QuickTime) is to use Format Factory to convert the video into a different format. It may help to view some of the video, but there may be stuttering, that is if it even can be converted. I've had similar issues with mp4 files that were recorded from a cell phone.
It played better after converting, but the video still stuttered and sometimes froze while the audio played fine. With this type of issue, not much can help because it was an encoding issue by the device that created the video.