I’ve seen Gate Guardian trigger for itself on a dying Kerberos, yes. But, the last spot Warg more than likely did trigger Pack Master to get a Warg in the first slot, which then chained Gate Guardian (glitched) as each one revived to repopulate the team. Pack Master and all personal death-summons appear to resolve before Gate Guardian is checked, so if the last remaining Warg summons another Warg via Gate Guardian, the first one should (hopefully) not then also trigger Gate Guardian because its check window has already passed. If the last one triggers Pack Master, it still has to check Gate Guardian and does so one at a time, down the line, for anything either currently alive or currently dying.
Another possibility is that each slot can be glitched more than once to allow multiple Pack Master triggers for a single Warg death, but this is not something I’ve personally seen.