Is Nick judging Gatsby for his ability to lie and commit to the Gatsby alter ego or is Nick admiring this skill? How do you know?

I think Nick is, in the end, impressed with Gatsby's skill. Gatsby, like Nick, is an outsider yet able to permeate the filthy rich society of East Egg with his own sense of style. He has a class that the others lack. Gatsby remains an enigma to many and although Nick understands Gatsby's frailties better by the end of the book, he is still in awe of the Gatsby he has built up in his mind:

“They’re a rotten crowd,” I shouted across the lawn. “You’re worth the whole damn bunch put together.”