Gems can also survive in a vacuum, and would easily be able to board the SD's via their vastly faster ships. Able travel between galaxies in less than a day. Gem ships are stupidly faster than Sw universe ships. This battle will not come to ship to ship, thats the only advantage the empire has, and the gems will know it due to sapphire future vision. People are under estimating the gems here. Thrawn has no knowledge of the Gems to start, but will have no language barrier and will be able to send and receive transmissions. If the conflict lasts too long, Gem reinforcements will arrive in a week. Thrawn desires to retake control of the planet and preserve it's natural resources and civilian population, so fire and brimstone tactics and WMDs are not preferable in a large scale. Most large cities are controlled by Gems, but the native population still exists and the ecosystem is still habitable. They also have a few dozen warships and light cannons on the surface.

They have 5k rubies, 2k Jaspers, 2k Amethysts, 1k assorted low level gems for maintenance(peridots, agates, topaz, bismuths, pearls, ect) a handfull of Saphires, and one Lapiz. On the planet, the Homeworld Gems, led by Blue Diamond, are using the natural recourses to create new gems. He has with him an Imperial Star Destroyer, three frigates, and the 501st legion(essentially, a little less than 10k ground troops IIRC). Grand Admiral Thrawn is dispatched to a world on the outer rim to settle a reported conflict.

Homeworld steven universe