Our (Portal Team's) motivation for adding GateIn 3.6 to stacks.yaml is that GateIn 3.6 would appear in JBoss Tools as a full-fledged runtime. As reported by manderse@redhat.com GateIn is the first project to go this path, so JBoss Tools team needs to check as early as possible if it is doable at all, see JBIDE-14763.

I hope obstacles on the side of JDF for including GateIn 3.6 in stacks.yaml?

I'd try to prepare a patch now. I'll do it analogically with JPP 6.0 which is already there.