After almost surrendering to the problem, and waiting to get an answer from other users in here, I tryed another possibility…
Changing the page-template for the default additional option in Gantry4 (Contact Form) displayed the form without any problem! (with a contact form at the bottom of course).
So I just made a copy of that page-template and just changed the title and deactivated the load of the contactform leaving the mainbody empty like this:
echo $gantry->displayMainbody();
Don’t know if it’s the correct approach but that solved the issue.