Manual installation along with having the GUI interface hooks into pfSense is extremely hard to do. It requires hand-editing a number of critical files. However, even if you did that, the new Snort PHP files won't run on 2.0.3 pfSense because they call and use system features that are only available in pfSense 2.1.x and higher.
So the short answer is you can't have the GUI with the current Snort PHP package on pfSense versions prior to 2.1.x. You can manually download and install the old *.tbz package, but you will need to use Snort exclusively from the CLI (command line) like you would if you installed it on a plain-vanilla FreeBSD 8.1 machine. You will have to create the snort.conf file by hand, download rules by hand, and start-stop Snort from the command-line.
Bill