If you set the filter to "mobile military" in the galaxy map, then just hit Q, it's very easy to see where all your mobile ships are to then gather them. Generally it's easy to just zoom all the way out and drag-select everything on the planet. The situations in which you are likely to have significant forces spread across more than 2-3 planets (that you would want to gather together at once, anyway) are pretty low.
There's also a Find interface on the sidebar for if you need to get all of your fighters (or whatever) from the galaxy to quickly gather at a single planet, for instance. There's a lot of power in those two commands in general, and usually if you're spread across too many planets with your mobile forces, you're probably doing something else wrong (not gate raiding enough, not using turrets -- which are more powerful, though stationary -- effectively enough, etc). Generally speaking there's almost never an advantage to having significant mobile forces on more than about three-four planets at once, even if you control 20+ planets.
