Hi there oscard, welcome to the forum.
From what you've said tourist visa is the only way.
Be sure that between you you will be able to show sufficient funding for the whole duration of the trip.
Also it's very important that your g/f is able to show strong ties to her homeland and good reasons to return home.
The ECO will naturally be suspicious that there is a romantic connection and that your g/f has no intention to return.
Sorry for such tough words, but your priority is to prepare you application as well as you can for this. Just be honest don't try to fool immigration.
Good luck
