No. It's not.
There's a huge difference between what you can do with BnSBuddy's XML editings and the GCD bullshit you're asking.
The most you can do with BnSBuddy is to change the order in which skills are done (or remove them).
You don't need BnSBuddy for that, but it is made easier...
You might gain some DPS, but, you don't need to do this.
Having you do the skills in the order you want is something you can do with absolutely no modifications at all...
Would be pretty fucking stupid to say that that's cheating...
Now ...
Editing GCD changes the timings of the skills, so you can do them faster, and produce more DPS compared to others that do not do these changes.
That's the definition of cheating!
You get an unfair advantage over someone else...
So, no, I ain't cutting you any slack.