afaik changa is nor so good in a GVG as the carbonizing of the herb seems to be a bit more conducive to vaporizing the harmalas. whereas the convection vaporization of the GVG gets the dmt vaped nice, without roasting the herb, but leaves the majority (if not all) the harmalas in the changa mixture.
granted i don't know this firsthand, it is what i have heard from multiple sources, however.
my recommendation (as always) is changa in the bonga.
its the simplest, easiest way to reliably dose with the molecule (maybe not the best methodology, but my personal favorite)
some prefer to drink a MAOI brew (or take an oral dose of pharma or what-have-you. sublingual is also farily common, as i understand it.) before smoalking dmt.
if you want the best of both worlds you could pre-dose with harmalas (in your preferred manner, be it smoking or whatever) and then smoalk changa on top of that (you would probably want a harmala-light blend for that).
of course you could just as easily do enhanced herb(i.e. changa without harmalas).
it really all boils down to personal preference, so the best way is to do everthing in small batches and play around with it until you find what you like most.
My wind instrument is the bong
CHANGA IN THE BONGA!
樹