Don't know if this is usfull, or how know it is.. but I just noticed this today!
When a gremlin lays a bomb.. a red ring apears marking it's radius, so we know where not to stand when it goes off.. That is common knowledge..
But what i hadnt seen befor is the second red ring that appears inside the radius ring.. this one starts small, and grows.. and when it is as big as the radius ring, the bomb goes of..
As Pupu said, all bombs have an outer ring to mark the radius and an inner ring to indicate the fuse.
It's important to notice with Gremlin bombs, as you realized, but also with player-placed bombs. To work effectively with a bomber teammate, try to lure or push enemies into the blast radius before the bomb explodes and be careful not to knock them out of the bomb blast. For example, if you are using a sword with a significant knockback, swing it at enemies outside the bomb radius and knock them into it. If you're using a gun, kite the enemies into the blast zone. That way you will be able to maximize damage against mobs and work with your bombing teammates instead of merely alongside them.
Of course, don't rely on the gremlins fuse if you are even the slightest bit laggy, or you could find yourself in an...
Explosive situation.
YEAAAAAAAAAHHHHHHHHHH!
We won't get fooled again...
Damn, can't believe I never noticed that. Thanks!