Have you looked at line 31 in the gantry.class.php to see what it is
trying to treference?
You haven't said which version of Gantry you are running so I can't
look for you.
It is possibly one of the following (in order of probability). Try
these one at a time and let us know which one worked:
1) php is not installed correctly. Add this line to the top of
the .htaccess file:
AddType x-mapp-php5 .php
2) Access permission: check that the required files are set to 644 or
655 depending on your hosts settings and the template folder and sub-
folders should be 777.
3) Ownership Permissions - the ownership of the file is not the same
as the ownership of the site. Activate the FTP layer or ask you host
provider to switch on CGI-PHP
4) Corruption of the file: try reuploading the offending file(s) -
reinstall Gantry