The subject pretty much sums it up - does gmdate() - or any of the other date() functions for that matter - get its time information from the user's own computer or from the server hosting the php file?
If it's not getting the time from the server, is there a way to do this? I need to lock a record by embedding a "last edited" time, and I don't want this to be screwed up by users who've set their clock wrong, etc.