Idioms are a very tricky subject on the GMAT. As Ron said, there are some idioms for which multiple forms are considered correct; "named for" v. "named after" is one of them. The GMAT will never ask you to choose between those two.

There are other idioms for which multiple forms are correct, but only in specific contexts. Take "known as" v. "known to":
KNOWN AS takes a noun: You are known as a GMAT conqueror!
KNOWN TO takes a verb: You are known to conquer the GMAT!
You need to know the context to know which one is correct.

Theoretically, idioms reflect the common usage of the language ("named for" and "named after" are both commonly used forms). The trouble with idioms, though, is that there are some for which there are several commonly used forms, but only one that the GMAT considers "correct." Take "different from" v. "different than." In my daily life in the US (speaking with friends, watching tv), I probably hear "different than" about twice as often as I hear "different from." But on the GMAT, "different from" is always the form used.

The moral of the story is - idioms are tricky to deal with, so try to avoid dealing with them if possible! Analyze all other grammar and meaning issues in a given SC sentence first, and only address idioms if they're the last split left (or if you're very sure of the correct form).