Dear Deacon Nikolai,
It is not possible, and in no way is it provided in the Church Canons, for a fellow-bishop, or Metropolitan, to "retire" another bishop.
My understanding is that Abp Gregory was never "excommunicated" by the ROAC Synod, but rather, "no longer in communion" with the ROAC (V) There is a difference in that the latter, it seems to me, is more severe.
Please clarify the Official ROAC determinations against Abp Gregory if you can.
Sbn Nathaniel