I agree with @Madalyn_Destafney's take on this and would add one question you should answer first: Are you comfortable with all the comments in Google docs and not with the project/proof in Workfront?
If you ever have to go back and cover yourself and reference a comment, it won't be as easy as just going to the project in Workfront. You can always use the Gdocs/WF integration but if the doc disappears from Google for whatever reason, you're out of luck.
I'm assuming you're using sanctioned company Google accounts, right? If not, there may be bigger problems.