The count that pops up is how many you collected in their town when they pop it. It doesn't tell you how many you lose if any. At least not that I've noticed yet
They don't steal from your stock of goo to give to your friends. Your friends who release gremlins into your town will get some goo from the gremlins released, but you won't lose any. However, you will get 2 goo per gremlin squashed, so I always squash the ones in my town + to make room for more gremlins from friends.
Gremlins carry 4 GOO. A player with a gremlin in his town has 4 hours to tap it. If that player DOES, they split the GOO (2 apiece) with the neighbor who left it. If the player does NOT tap the gremlin, the player who left the gremlin gets all 4 GOO.