Base form GT Goku is likely = to Buu Saga ssj3 Goku.
Why? Because he has trained and was on par with a fully trained Uub, who is the reincarnation of Kid Buu.
Base form Goku also shattered a dimension, somewhat comparable to Super Buu and Gotenks' ROSAT feat.
GT Goku pre-ssj4 beat Rildo, who had a greater ki than Majin Buu.
Also base form kid goku who was injured and had no energy left kicked away an Earth destroying attack by Yi Xing Long.
Now ssj1 is 50 times base form, ssj2 is 100x base, ssj3 is 400x base, (all in Daizenshuu 7)
There is no official multiplier for ssj4, but we know it is obviously bigger than 400x base. Every ssj multiplier is at least double the previous one, so ssj4 would be 800x base form minimum.
Also remember that turning into a child did not diminish Goku's power level, it only obstructed the Shunkan Ido technique and made ssj3 more draining.
And the ssj4 transformation was so powerful that it undid the magic of Red Shen Long's granted wish made with the black star balls, and the black star DB's are said to grant any wish with no power restrictions.
This being said I would conclude that ssj4 Goku is at least 800 times stronger than Buu saga ssj3 Goku, or (400 x 800) 320,000 times more powerful than base form Buu saga Goku.
Now in terms of destructive power, GT is a direct sequel to the Z anime, and in the Z anime Kid Buu was depicted as destroying galaxies (also backed up by manga statements) and I have shown how much stronger ssj4 Goku would be then him.
So long story short, he is much much more powerful than superman, lol.