I had the same problem! and According to the FAQ:
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4.2: I've unpacked Gallery and run the configure.sh script, but when I go to the setup directory I get a Server 500 error. My Apache error_log reports php_value not allowed here.
This is because your webserver is allowing some .htaccess overrides, but not others. The Gallery setup script tries to set some PHP settings and requires you to either set AllowOverride Options FileInfo or to disable overrides altogether. See the Apache docs and this tutorial. In a nutshell, in your httpd.conf file you need a directive like this:
AllowOverride Options FileInfo
If you are unable to modify your httpd.conf file, you can edit the setup/.htaccess file and comment out any lines that start with php_value.
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However, this didnt work for me until I removed the top two lines, one beginning with "php_value" the other beginning with "php_flag"
now it appears to work.. :smile: