If they're a group that's used to playing larger races, expect a few rough games during the teething period as they adapt their tactics. While I've never done gnomes, I have been in an all halfling group and it took a few games before we adapted to the realities of being three feet tall. Lower average damage for the fighter types due to small weapons, lower jump and movement scores that limited our ability to move around in combat, being very vulnerable to grapple and bull-rushes. We had to unlearn a bunch of habits from previous games, and learn to use our advantages (better hiding and stealth, concentrated missile fire).
Make a point of throwing them against small opponents early to ease them through the transition, and make use of the occasional medium opponent to highlight the issues raised above early enough that they start thinking of them before medium-opponents are a regular occurace.