My gf thinks my dad is using me ?

I mean it's your dad?? I don't know about you, but if my mum asks me to do something and I'm able to do, then I do my best to help. Your dad is getting older, he needs the support. I don't understand what your girlfriends deal is to be honest.

Your gf may believe that your father is taking your help for granted or in the habit of exploiting you by making far too many demands and being a nuisance.
Talk to her and see whether you agree with her opinions.

It's not like my dad is asking me to do stuff from time to time, it's far more than that. My dad's 80 but he's not your common 80 year-old he still works.

I accept to help him because he's my dad, but I've forgotten about myself in the process, and I think that's what's troubling my gf. She here's him asking for stuff like driving 3 hours to and from his just to turn on the electricity and heating when he could ask other people or simply do it himself. Like for his car.

I have my own life my own problems that I need to focus on.

If the roles were inversed and I had asked him to turn on the heating, he would rightly say no that it's up to me.

Wow your dad works at 80 what does he do?. How does he manage that? I had the impression that he was frail and needed support. I am confused where you had to turn the heating on. Is it to keep your dad warm or something else?

He's a builder. He has several jobs on the go. He does everything from masonry to plumbing. Honestly, you would think he's in his late 60s.

I had to turn the heating on at his second home here in France. He's coming over with my sisters for a few days.