The X7xx taillight assembly won't fit the X5xx, it is larger. The lens is a red/clear joined piece, the X7xx also has a 'backup light' that comes on when you press the reverse pedal, or you can turn them on with the headlight switch if you want to use them like a worklight.
I can't really see much use for taillights on a GT. What I would like to have seen in their place are flashing amber lights. I have to mow along a highway in front of my house for about 350 feet. I don't like to throw the clippings out in the road, which means I have to mow with my back to traffic. Lots of heavy truck traffic, gets kind of scary when you hear them coming, even though the speed limit is 45mph.
I can see a use for headlights, but what's going to be following you that would need to see you, in the yard, at night? I guess reflectors are good, so you can see that you forgot to move the GT out of the driveway, and don't run over it, but I wouldn't be out near a highway after dark at any rate. Taillights are pretty much the same, but I would like flashing lights.