It really depends on the bike. On most bikes the clearance at the chainstays is probably the limiting factor, so 2x or 1x is not really going to make a difference. If you have huge chainstay clearance and a slack seat tube angle then 2x could limit your tire choice.
My $.02: It all depends on the geometry of the frame and the design of the front der. The SRAM Red eTap front der. has a battery which intrudes on the space of the rear wheel. My Swiss Cross Canti with a SRAM eTap front der. could accommodate up to 40mm gravel tires like the Maxxis Ramblers. Anything wider would probably be a problem. JOM of Gravel Cyclist found the same issue in his review of the eTap system which he discussed with SRAM tech. people at a recent trade show. The SRAM response was that they felt that most gravel bikes would use 1x systems. Not a very encouraging reply from SRAM. My new custom steel 44 Bike Huntsman was designed to also accommodate up to 40 mm knobies with the eTap front der. However, the frame will take up 50 mm 29er tires in a 1x set up, and perhaps 45mm tires if I switch to Shimano Di2. The Di2 front der. gives you somewhat improved rear tire clearance. If you are planning to upgrade to an 2x electronic gear set, and want the best rear tire clearance --- then I would recommend Di2. If you can live with 40mm clearance, then SRAM eTap is a great system. Mine has been flawless for almost a year of daily gravel and road riding.
