Shimano GRX (or similar) and a flat bar - possible? I've got an old MTB hardtail GT Avalanche 1.0. Now it has SRAM X7 (3x10) crankset (but used in 1x10 setup with a nw chainring). This bike is my commuting and "road" bike.

With 36t chainring on the front I can't seem to ride as fast as I hopefully could with a larger one. So the question is: is it possible to install a road (or road-ish - like GRX mentioned in the title) double crankset and a derailleur on this bike? I want to leave flat handlebars in place in order not to make the geometry too strange.

Or, can I somehow use a double derailleur with a 3x crankset? Since I never use granny gear on this bike and don't like 3x mechs.
The GRX is theoretically possible. Or at least better than road ones, bucause it s specifically designed for larger tyres so it gives more clearance for wider chainstays. It moves the chainline by 5 mm outward in comparison to road cranks.

Also, the 46-30T chainrings on FC-RX600-10 give a bit more clearance than compact road chainrings.

Still, your MTB frame's chainstay could be too wide. You will have to measure it or try it somewhere.

As for the flat bars, the Tiagra flat bar shifters SL-4700 should be compatible according to the compatibility chart.