You shouldn't have to change 'runtimepath'--the default setting should work in all but some unusual cases.
If you set 'runtimepath' as you show above, yet you put your SnipMate files under USER/vimfiles/plugin or USER/vimfiles/ftplugin, Vim isn't going to find them.
I would:
- put configuration settings in _vimrc rather than _gvimrc,
- not set 'runtimepath',
- make sure the SnipMate files are where they are supposed to be.
Then if it still doesn't work, execute
:scriptnames
to see all the runtime files that Vim actually loaded and make sure you put the SnipMate files someplace Vim is looking.