In the latest Gorilla update, we've launched the Allocator Node, which will cover this much requested functionality. It's available today and you can read more about it here: https://support.gorilla.sc/support/release-notes#releasenotes

This is now on the internal development map! We'll be creating an Allocator (sensitive to attrition)

You'll be able to define branches that are set to have a fixed number of participants. Then, incoming participants will be randomly allocated to the branches until all have had the required number of Completes. If a participant on one of those branches gets rejected, they'll be removed from the Allocators set for that branch and a new participant will be set through that branch.