I have been getting a lot of 500 internal server errors. I have not exhausted checking possible causes of the issue and had not considered Gantry before now. However, I ran across this post from the developer of OnePage Checkout for VirtueMart. ww.rupostel.com/phpBB3/viewtopic.php?f=5&t=321
He had not contacted you because he wasn't sure if it was Gantry or the Gantry user he was helping that was responsible for the particular code he said was problematic. I checked one of my client sites and discovered the code he mentioned in several places including the one he mentioned. I am not a programmer so I am passing this along in case it helps.
I managed to recreate this issue on my test web site with Diametric template so I can confirm that this is real issue with latest version of Gantry and Diamteric template overrides. Do you also use Diametric template?
Please try to install version of Gantry 4.1.2 (previous version) until this is resolved. I already notified our devs about this issue