I took a quick look at my bike and you should be able to handle just about all routine maintenance with the following:

Set of T-handle metric Allen wrenches (Craftsman Pro Series is very good, you will use mostly the 4,6,8 and 10 mm sizes) set of metric 6-point open/boxend wrenches 24mm socket for rear axle nut. Flywheel pulling tool for your year bike (your Gas-Gas dealer probably has these) a good 12" crescent type wrench (buy a GOOD one, the cheapies will round off the aluminum fasteners) spark plug wrench. In addition, you will need the usual oil measuring cups, usual shop tools like screwdrivers and channel lock pliers, oil drain pan etc.

Jon