"I've actually modelled by myself in Bryce this spaceship that apper as a tiny - and not so clear - spaceship in background of a huge spaceship in a Gambino illustration  (as you and everybody can see)."

That's right, you've taken a design by Mr Gambino and made a commercial product out of it without credit. The size and emphasis of the original was enough to catch your eye as being suitable for a saleable item; and enough for you to quite closely match the shape and details shown.

"If I model a WWII Spitfire airplane or a WWII German tank ... is GOOD, but if I model a spaceship that is just a DETAIL in a illustration of Fred Gambino is BAD???
WHY???"

Because the former are models of real historical objects whilst the latter is a straight lift of someone else's artistic design. If you don't recognise the difference, perhaps you are unsuited to the field of commercial art.

"Are you shure that your reasoning is really working or do you have a personal problem with me?"

My reasoning is clear, as you have confirmed in your response. I'm not aware of having a personal problem with you other than the fact that I don't like plagiarism.