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.
- 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
to see all the runtime files that Vim actually loaded and make sure you put the SnipMate files someplace Vim is looking.