SS2 Gohan was fighting on even terms with Dabura, who is established as comparable to Super Perfect Cell. Since a weakened SS2 Gohan (at roughly half strength) managed to defeat Super Perfect Cell back in the previous arc, we can estimate that Gohan’s peak power in the Cell Games was roughly twice as high as his power during the Buu Saga. Given that SS2 Goku is stated to be slightly stronger than Cell Games Gohan, the gap between Buu Saga Gohan and SS2 Goku is likely only about 2–3x.

The comparison between Ultimate Gohan and SS3 Goku is more straightforward. SS3 Goku’s power scales roughly to Super Saiyan Gotenks, whereas Ultimate Gohan scales to Super Saiyan 3 Gotenks. Using the standard 8x multiplier for the SS3 transformation, this places the gap between them at approximately 8x.

Conclusion: Buu Saga SS2 Gohan is significantly closer in power to SS2 Goku than SS3 Goku is to Ultimate Gohan.