Gat art alongside metamidion: Yes very effective, as shown by SPQR a couple of times in competitive matches now.
Gat art vs other ships?
Longer range disable vs Blender fish,
More regular disables against Catfish,
Ability to keep Galleon broad side disabled in two to three shots (with a good gunner, three for left side.)
Disables on squid engines makes piloting for the enemy difficult and often causes them to collide with terrain or make mistakes in positioning (in my experience in both pubs and competitive)
But the Gatling mortar is very weak against mobula and Junkers as the disable for a Junker is 3 very precise, hard to take shots, and same for mobula..
Positioning: Crafek you've got it spot on, height is the ships requirement, thus making staying Lowe and sneaky a big risk with the time needed to raise altitude, and staying high in advance tough as you'll be easily visible.