Mary, I tried I a few things on that list and still no success. I am fairly new to linux so I appologize for not knowing too much about this.
Please try and analyze your log files and provide as much detail on what you did as you do not give us much to start with. Saying you did several things from that list does not tell us waht you did, not what the results of those actions were.
I would seriuously recommend looking through your log files and see if you find any clues there, the first to look at ar /var/log/messages, /var/log/httpd/access_log and /var/log/httpd/error_log.
My first guesses are a PHPBaseDir restriction, which has a section in the FAQ in the wiki, I believe. You can also temporarily enable the display error function so errors are posted on screen (although I am not sure if this goes for gallery2 as well). All I can tell you is that I have gallery2 running for a few years without problems, so it should be possible. I even have it integrated with a wordpress blog, all from the time before they were packaged in contribs.