Post a custom Forge event to the EVENT_BUS when a gate is merged (SGBaseTE: 834). This would allow for addons that react to gate creation and removal. If the event contained the gate's address and whether the gate was merged vs un-merged we could build addons that spawned mobs from the gate's position, add the address to loot elsewhere, etc.
Not sure about your PR policy - I could probably do this if you're currently accepting PRs.

This was pulled. Thanks for taking the time to do this. Sorry it took me nearly a year to get around to this.

I would accept a PR for this. I'm about to start working on this again.

Oh, I see you did a PR, I will review.