Another way of playing a G chord is to use middle finger on the A string, ring finger on the low E string, and pinky on the high E. If that sounds like it makes no sense, try doing it and then changing to an "open" C chord shape.

As for the "This is how musicians play a G chord" comment, the salesman was talking out of his hat (I'm being polite). "Musicians" will play whatever chord voicing using the most logical or convenient fingers to fit the situation. All he was showing you was a different way to play a G chord, and not "The musicians' way".

I've been playing guitar on and off for a few decades, and can think of several different ways of playing what would fit as an open G chord, using anything ranging from 1 to 4 fingers, and sounding 2 to 6 strings (although some might argue that it isn't a true G chord unless it contains the Root, 3rd and 5th notes of the scale). I sometimes choose one voicing and fingering on a whim, other times because it sounds "right", and other times because it's the easiest to use to fit between the previous and next chords/etc.

Enjoy your new guitar, and your journey to hopefully becoming the guitarist YOU want to become.