Though it's never explicitly stated, the fact that Gus appears so flustered when Hank asks him "Is Gustavo Fring your real name?" combined with the fact that Hank is shown to have the right instincts most of the time, to me indicates it's probably not his real name. Or is that too much of a leap?
That is a safe assumption. From the Wikipedia article:
Gustavo Fring is established as an alias, as neither the DEA or Mike can find any records about him prior to his arrival in Mexico.
The Wikipedia article about the character is quite long and provides some extra information including the possibility that he was originally part of the Chilean dictatorship of Augusto Pinochet.