It's an affectionate synonym or corruption of 'girl'.
http://www.word-detective.com/2011/08/gal/
see also 'gel' (pronounced with a hard g) for the upper-class variety that unsurprisingly didn't make it over the pond :D
I think it depends on context- when used with close friends who like the youthful, almost archaic connotation of 'gal', why not. I see posters for 1940s/50s themed parties called 'guys and gals' and a female friend of mine and I have used 'alright gal' and 'alright lad' as friendly greetings for as long as I can remember. I wouldn't want to be the arbiter of what is and isn't acceptable across the board in all situations for that one. However, in a professional environment or when talking to strangers I avoid 'girls/gals' as a counterpart to 'guys' becuase of the infantilisation/junior element to it, and I can see why it drives people nuts. I avoid 'girls and boys' too as my staff/colleagues are my professional peers deserving of respect for their skills, not my students or my children.