When an approach chart says RNAV1 GNSS required, what does this mean exactly? Is a GNS530 enough? GTN750? G1000? What features does the aircraft need to legally fly this approach?
This question is a bit of a can of worms if you’re not an AV geek. 
And will lead you down a google rabbit hole of SIDs and Stars, RNP vs RNAV, RAIM, etc…
Someone more experienced than me can chime in I’m sure. But there are differences between an RNAV1 and an RNP1. Whether the aircraft has RAIM or not is the reason for the GNSS warning. You can fly the procedure without RAIM. (Receiver autonomous integrity monitoring.)
But yes any of those Garmin navigation systems can fly an RNAV (gps) approach
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Which airport and approach? You can generally fly using GA FMS if it doesn’t say AR (Authorization Required).
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GNSS is an umbrella term that covers both the US GPS system and the Russian Glonass satellite systems. RNAV has been around since before GNSS. It used signals from multiple radio navigation aids to create a waypoint. With the introduction of GNSS, aircraft navigation systems would use the satellite system as its primary data source and back it up with radio navigation aids.
What the note on the approach chart means is that you can’t fly the approach unless you have a valid GPS/GLONASS signal. You can’t fly it using radio navaid created waypoints.
The good news is, none of that matters in MSFS. You always have access to the GNSS satellite system. Any of the navigation systems you listed allow you to fly the approach.
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Here was the one I was trying: ILS/DME Z. I utterly failed, not only did I mess up my DME arc and the hold at WITBY, but I also got carb icing and crashed a few meters from the runway, lol.
It’s all good though. I’m here to learn and I feel next time I’ll do better.
Bonus question: Can you enter a DME arc at any radial? Or should you enter at the radials marked on the approach plate? I notice the Garmin has these programmed in as transitions, so it got me wondering. Last night I just used vectors for transition and flew it (or at least tried to) manually.