Let's break this down fr fr. The Luffy and Doflamingo fight can only fall under one of three categories:
1. 100% Doflamingo is stronger than 100% Luffy.
2. 100% Doflamingo is equal to 100% Luffy.
3. 100% Doflamingo is weaker than 100% Luffy.
Let's take a look at what each of these scenarios imply for the canon.
1. If Luffy is stronger than Doflamino with both at 100% and was unable to finish off Doflamingo when both were weakened, that would mean Luffy took more damage than Doflamingo did because he was reduced to a state of inferior health and ability than Doffy(Doffy outlasted him).
If Luffy took more damage than Doflamingo did, then that would mean that Bellamy's assault on him was greater than Gamma Knife. That would mean that Bellamy's assault on Doflamingo would push him closer to death than Gamma Knife did.
2. If Doflamingo and Luffy are equal in power, then it means Luffy still took greater damage, as he was rendered in a state of inferior ability due to Doffy outlasting him. Basically the same as above.
3. If Luffy was weaker than Doflamingo when they were both at 100%, then that means Bellamy's assault could be anywhere between weaker and stronger than Gamma Knife. This makes this scenario the most likely one, as Bellamy is definitely ain't as strong as Law's strongest attack.