If you always keep the genuine .orig.tar.gz file in the parent of your working directory, I think there is no need to change your workflow. But if the file is missing (e.g. after a fresh `git clone`), `gbp buildpackage` will recreate it. Otherwise, one would have to download the .orig.tar.gz from somewhere (as I did, from mentors.debian.net).
There is no need for the sponsor or the occasional uploader to add him/herself to Uploaders; any member of the team can make a so-called "Team upload" if needed. The semantics of Uploaders (AFAIC) is the actual maintainer of the package.
I will upload ocaml-gen soon. Thank you for your contribution.