You haven't supplied nearly enough information for anyone to tell you one way or the other what you should do.
However, I will give you some questions you should be asking one another before you make the decision.
1.  What EXACTLY does your girlfriend wish to get from this?
2.  What level of sexuality does this activity satisfy for both of you?
3.  How do YOU feel about it?
4.  What are YOUR expectations in regards to this?
5.  What is the reason why the intimacy must be restricted to tickling only?
My guess is that when you answer these questions, particularly number 5, you won't find it necessary to ask anyone for advice.
Then again, perhaps this thread is a thinly disguised solicitation for a tickle partner, hence the included pics?  Hmmmmm.  Stranger things have happened.
In any event, Welcome to the forum, have fun and best of luck!  🙂