I saw the fight between Mutado and Gigant Zamasu vs Vegetto SSJB and I can say that Vegetto SSJB did not fight 100% at first, but something relatively close to 100%.
At the beginning of the fight between Gigant Zamasu vs Vegetto SSJB, it seemed that both had the same strength, but Gigant Zamasu was completely slow, then Vegetto SSJB launches a very lightly charged Final Kamehameha and finally Vegetto SSJB appears a blue aura on his arm and hits Gigant Zamasu even harder.
With all the above mentioned by me, I would say that SSJB Vegetto only uses his 100% at the end of the fight. I have Vegetto SSJB a little stronger and a lot faster than Gigant Zamasu.
If we make comparisons of power differences with other characters, a good comparison would be: If Vegeta SSJ (Arc Android) equals Vegetto SSJB (Anime), then Future Trunks SSJ (Arc Android) would equal the strength of Gigant Zamasu and the Android # 20 (Initial) would equal Gigant Zamasu's speed.
Vegetto SSJB (Suppressed vs Mutated Zamasu) = 5,000
Vegetto SSJB (Suppressed vs Gigant Zamasu) = 6,000
Vegito SSJB (Final Kamehameha) = 6,553.6
Vegeta SSJB (FP) = 6,250
Mutated Zamasu = 4,800
Giant Zamasu (Strength) = 6,000
(Speed) = 3,840