For one thing, the pieces from Gap, H&M, etc, are of a visibly lower quality than authentic Isabel Marant.
In some cases, yes, though (having tried on both the Marant and Gap versions) I have to say that in this case the Gap was the winner (IMHO)...and that's including having found the IM dress on sale for not that much more than the Gap.
Going off-topic (as far as IM is concerned)...but to follow up on luckyme's comment...isn't it more important that you like what you bought (regardless of where it's from)? If you love the IM version wear it to death and who cares if people are wearing Gap "copies"...

I love your shirt.