Ssj4 Gogeta was capable to nullify and purify a magnitude of energy which was said to be universal via chain reaction.
Ssj4 Gogeta was thus ranging in a level of power between multi-galaxy and low universal.
On the other hand, first appearance SSG Goku was capable to tangle with a suppressed God of Destruction, putting the fate of the whole universe on the line with just few physical hits.
It's true that Beerus was responsible as well but, in order for him not to atomize SSG Goku, it has been necessary to low his power in order to more or less match that of his opponent.
So, the level of power of first appearance SSG was capable to potentially waste the whole universe.
And that happened because Goku was not capable to fully control that new, immense power at the time.
The transformation basically brought Goku's power from around Boo saga Ssj3 level to something probably greater than Ssj4 Gogeta's output.
So yes, GT got surpassed when SSG Goku was introduced.
And the chain is:
Current Goku >>>>>>>>> BoG SSG Goku >= Ssj4 Gogeta >> Omega Shenron > Surpassed Limits Ssj4 Goku > SD arc Ssj4 Goku > Baby saga Ssj4 Goku >>> GT Ssj3 Goku >>> GT base Goku > Boo saga Ssj3 Goku