The Instant spy actions for GMs are color coded and correlate to the non-GM spy color coding assignments..
I have numerous times effectively used Mil spies to temporarily damage a railroad to slow their rapid re-deployment to defend against me. It's also useful to disrupt production of a battleship, railgun, etc if you can damage the harb/rail or factory.
There are ways to help reduce the chances that their Mil spy will target something important. I'm sure you can deduce how to do that.
I don't us GM spy actions that much except for one. An opponent could raise a level 2 fort to a level 5 in a matter of seconds with GMs if I am coming at him with a large stack and he is unprepared. I would never know it until I have gutted myself easily turning the tide of war. That's Bovine Scat in my book and Bytro refuses to address such things reasonably so to counter that I may do an instant Intel Spy on that prov to reveal it's fort level. It also comes with revealing troops there and in nearby provinces which I don't like but I must know if they raised that fort level instantly. It really does suck that I have to do it.
When you are using spies you really want to use intel spies often and rarely use the other two. It's very dangerous for spies to take overt actions of sabotage.