2005 Elantra -- AAAAAUUGH!!

[Brian Nystrom]

Is not the GT a more sporting version that has a higher gear ratio for
faster acceleration and therefore slightly less gas mileage?

In a word, "no". The transmissions are the same in the GLS and GT, as
are the EPA mileage ratings. Where there is a big difference is between
the manual and the auto. Although the EPA ratings aren't much different,
in the real world, the manual can exceed it's highway rating whereas the
auto seems to struggle to meet its. City driving is so variable and
driver dependent that it's hard to say what difference there is, if any.