Yes. Use it. Edit it however you like. Redistribute it, modified or unmodified. Sell it. The GPL explicitly allows all of these things, provided that any such distribution is done also under GPL.
While the GPL requires inline-documented author attribution and copyright/license information must remain intact, there is absolutely nothing wrong with removing public-facing “credit links”. Anyone implying otherwise is acting contrary to the Free Software philosophy from which the GPL itself is derived.
If a developer wishes to compel users to retain a public-facing credit link, then the developer should release his work under a license other than GPL. But to release a work under GPL and then attempt, through appeals to moral propriety, to restrict the very rights granted by that license, is an act that is itself morally repugnant.