I don't explicitly know about GRT, but something I think about often is the "bus factor" of companies I'm buying from. How many people have to get hit by a bus for the company to cease to exist? E-AB is incredibly grassroots and hobbyist-driven, so for a ton of suppliers we deal with, the "bus factor" is 1.
A quick perusal of old VAF threads will show you builders that are no longer with us, individuals who came up with neat parts or solutions for their own project and spun up a boutique business for a few years before evaporating. So many that disappeared due to age or time, and even some great ideas that never took off or suffered infant mortality after a promising start. I stumbled upon two more defunct companies today alone - vanished without a trace except a user name and some broken links to a domain that hasn't existed in perhaps a decade. Is GRT one of those? I sincerely doubt it, but somebody who is more familiar with the company should chime in.
This topic deserves its own thread but I'm hesitant to ask publicly which companies are one accident or illness away from vanishing.
I have been around long enough to see lots of changes and innovations in this area. My general advice regarding the original question goes back to what Paul said years ago as it still holds. Go sit behind a variety of displays. See what "works" for you. Choose that one.
Regarding the survivability of GRT (or others), one needs to look beyond just size. For example, many years ago, when GRT was a MAJOR innovator as a supplier of EFIS to the experimental community, Dynon was formed and had a few missteps when first out of the gate, BUT(!!!), that small startup went back to the drawing board and got it right. Many would have written them off if not for asking deeper questions. How many of you have new KING radios or EFIS systems in your EAB? And they were the so-called "800-pound gorilla". The founder/owner of GRT has *chosen* to not try to be a large company. His company, his choice, his life. But as a small (boutique even) company, he claims (to me) that they are doing quite well. I suspect that MGL may be similar. Regarding the 1-bus measure, I am sure that they, as others, have someone, who if they lost, would cause major disruption. Anybody ever heard of "The Woz"??? His company did "pretty not bad" even though it went away from what he focussed on.
So yes, the BIG one is Garmin. Chomping at them is Dynon. You won't go wrong with either ("Nobody ever got fired for buying IBM"), but make sure that whatever YOU choose is what works for YOU, and not us here on this forum, with all of our biases.
BTW, as someone who had a team responsible for "Human Interface Technology" and "MMI", after flying behind the latest Dynon, I give the kudos on usability!