Not exactly sure what you mean as a 'good gym'- 24 Hour Fitness has lots of equipment, not really 'hardcore' gym though- open 24 hours (obviously)- located in Grossmont Center
Parkway Fitness at the corner of Fletcher Parkway & Marengo is open 24/7- more of a 'real' gym- Frank Zane trains there
but if you want convenience...24 hour is good and tends to have sufficient equipment. i know a few pretty good bodybuilders that go there just to escape the b.s. and competition of the hardcore gyms.
and Regurgitator,
if you're going to be around the area 9/22, the 'Border States BB/Fitness/Figure Comp' is in pb- didn't mention it before 'cause you said La Mesa area- Nassar is guest poser at the contest... only a short drive from La Mesa