Depending on the amount of upgrades you want to do,
To start,
Like Spl. Durkee stated, a TBB
Better Hop-Up Buckling,
Spring Improvement.
If you are just starting in AS, let somebody with experience show you how to do the upgrades, don´t attempt the upgrades yourself, GBB´s are extremely tricky in comparison to your normal AEG, if the Tuning is not done correctly at a minimum it will lead to you having to tear down the gun again, and start from scratch, if things really go south on you, you may up destroying the gun.