I only mention if there is something reconizable about the card...for instance one girl drew a line around the head of a bird to make it look like a space-suit for a moon-themed card...I did the same thing on
this card and gave her credit for her original idea.
If it's a basic layout, and you use similar colors...there is no need to mention that you saw someone else do it first...there are only so many color combos and sketches out there. LOL
If you refer back to one specific gallery card a few times while creating yours, then yeah, mention it.