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 Max Rydahl Andersen 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.