Are plastic dryer vent fittings (not hose) allowed with gas dryers?
In general, yes, unless otherwise specified.
I wasn't able to find much information about the specific one you linked, but if you go to other home improvement websites, they tend to list more information, as well PDF spec sheets. Home Depot for example, has a bunch of these made of plastic, that all say they are OK for use with gas or electric dryers.
If there is a manufacturer phone number on the box or tag for this specific unit, I would just give them a call and ask.