Yeah, your idea won't work. You need to create a new membership *provider*, but it's not as scary as it sounds. You only have to implement the methods that GSP uses; leave the others empty. For example, if you won't be creating users in GSP, you don't need to implement the CreateUser method (just use an empty method body and return null).
One of the most important you will need to implement is the GetUser function.
I don't understand your question about "standard provider methods".
I advise making a copy of the SQLite membership provider class and modifying it to suit your needs. Start by deleting the contents of all override methods except the GetUser one (the one with this signature: public override MembershipUser GetUser(string username, bool userIsOnline)). Modify GetUser for your situation and start experimenting.
If you will be using GSP for reading membership rather than updating it, that might be just about all you need.
Roger Martin
Creator and Lead Developer of Gallery Server Pro