The fractal antenna inside the case seems to be pretty immune to such issues. That said, once you get outside that comfort zone and hook up an external antenna, then I’d say, yes, make some provision for grounding as it would certainly introduce that risk to the GTM.
On the other hand, given the itinerant nature of many GTM use cases, this might be something which calls for some flexibility. A GTM is typically not hooked up to the power grid. When used in relay mode, a GTM is usually not in close proximity to humans. For remote temp installs out among nature, well, lightning hits trees all the time. A. node slung up in a tree likely has limited potential to cause issues. Still, lightning is dangerous. Be careful out there.