Thank you for your answer Rolfy.
The original models use several textures on them, and as you know GG uses only one texture by object.
I actually used some original textures of the game, mixed with others (found on the web) in order to create only one big texture.
I understand I need to change all the textures parts from the original model... After that, I would be in legality !
PS : I work in this way because I'm not good designer but I like to create 3D objects... and I don't do that for money but just for fun, like many people in this forum I guess ...