Correct, but it is possible GRM will work (somewhat) in the default game.
1) Install GRM's "My Document" content. Do not install the "For your game directory" content.
2) Go into the Mods folder... remove all GRM folders except GRM itself... (some of the additional content might work, but let's start with basics.)
Go ahead and boot up GC3 and make sure "Enable Mods" is checked in settings.
If we're good... you'll get no error messages and you'll get the basic added abilities and my races/shipsets.
Remove the final GRM folder from mods... and remove my factions and flavor texts from their corresponding folders. You'll now have all my ships and the various art... but no ship-sets and no modded content whatsoever. You'll have to make shipsets yourself using the fleet editor if you want shipsets.
Or just... buy Crusade... in which case, most things will work.