Go, you might enjoy it, and you'll meet new people.
If you want to you could offer to share the cost of the petrol, but she invited you and would still have to pay if you didn't go!
A little gift for the hosts would be nice. What kind of party is it? Birthday, anniversary or what?
How about some pretty soaps or drawer scented paper? It's hard when you don't know them but you should ask your friend what kind of things they like, if they have hobbies or whatever?
It doesn't have to be a gift for the actual party reason, but something to leave for them to say thank you for inviting me/putting me up!
I think it could turn into a lovely girls outing kind of thing.