Same here, it just won't work. I've tried everything but all we get is a blank result, that is the {gallery}folder{/gallery} tag gets replaced by nothing, no images no errors nothing! I've tried 3 servers and all with the exact same result:
1 our account with fasthosts.co.uk, it seems this is an apache server with php fastcgi
2 my testserver running X/OS Linux (RHEL4 clone), apache and mod_php
3 my internal server running CentOS 5, apache2 and php5
All have the gd lib installed.
The 'funny' thing is we have the exact same problem with Easy Extended Gallery which is very simular to Simple Image Gallery Pro!