They play well together
Not to hijack the thread, (NOOB question) but could you discuss the integration to the non GRT electronics in the panel. I would love to start with a day/night VFR set up then upgrade, as funding permits, to full IFR. What other electronics are needed?.Xponder, nav/com, etc.
Great question.
The GRT EFIS systems, as well as all the others except Garmin are basically stand alone companies. Garmin is the only one that actually offers EFIS's, radios, certified GPS's, etc.
That being said however, GRT and all the other EFIS systems will play with all of the radio/transponder/GPS's very well.
For a basic VFR panel, you'll probably start out with a COM radio, an intercom or probably a COM panel (for future upgrade), a portable GPS and a transponder. The portable aviation GPS's available today will almost all play with the GRT very well. Down the road towards IFR, you'll want to get a certified GPS, by then most likely WAAS.
Transponders today are coming down in price. I replaced a broken King KT76A with a new Trig TT31 because it's ADSB compliant with a certified GPS, but is completely usable as a mode S transponder today if you input a certified WAAS GPS position into it. Trig and Garmin both make remote transponders, but they require remote mounting and a controller mounted in the panel, or thru an EFIS.
For COM, there are several basic radios but you may want to consider one that can at least MONITOR the standby frequency if your are going with a single COM now. The SL30 and SL40 Garmins do this as well as others. (Garmin's new COM/NAV's were recently announced to replace the 30/40). With the IFR panel, you'll look at certified WAAS GPS's with COM/NAV built in. Pricey but you get what you pay for. (the 430/530 and the GTN 650/750 come in here.)
Then when you get ready to upgrade you already have stuff that won't have to be replaced, just added to.
As for the EFIS, none of the above really affects it. You can add autopilots that are included in the EFIS, like GRT and Dynon, or, add stand alone autopilots like TruTrak. All of the EFIS systems work with different autopilots though, so just plan ahead.
This is a process that really needs to be thought out before you start building your panel. As always, things are changing so fast in the electronics world what you decide today will be changed by next week, but you can still plan upgrades in and save yourself a lot of headaches.
Good luck with your project.