I have 574 Brass spoils right now. I forget how many per Grade 5 you need, but that's still going to be a good amount I don't need to spend gil on later.

3.2 gear will probably have a higher chance to convert into Gear 5 materia as well, and once some of the Diadem changes come into effect I'll go back in and farm it more for tokens (only really going in there to gather for now)

Also, don't forget that Grade 5 cannot be overmelded, so you don't need a lot of them. Grade 4 will still be the ones you need to get a lot of.