TWIG version used by Gantry 5 is 1.42.5
You can always check by putting
<p>Twig version is {{ constant('Twig_Environment::VERSION') }}</p>
In a customhtml particle somewhere on one of your layouts and ensuring that your have the "process TWIG" box checked - the version will then show wherever you put the particle.
Regards, Mark.
The following users have thanked you: Gene Teigland