You can import ONX points to GAIA and vice versa, but you would never want to go the other way. Additionally, you can export each point as an kml and lay them out over Google Earth, which technically I think you can do with ONX too, but GAIA is better.
If you are used to the topo feature in ON X then you will want to use topo lines overlay + shaded relief - and what's cool is you have the ability to vary the opacity of each. I also like the "national map" b/c it has good topo and adds names of things. You can alter the opacity of various maps such as USFS 2016, GAIA topo, and Historic USGS maps and this will sometimes, as it has for me, reveal old trails or trails that are on one map, but not the other. This has, on more than one occasion helped me find some amazing brook trout waters and/or helped keep me from having to bushwhack the whole way. I had a buddy that sent me an ONX trail and it was literally 50 yards from an old abandoned FS road, but he didn't know it was there. His exact quote was "only do this if you are feeling adventurous. When I told him I found an FS logging rd that paralleled his DIY Bushwhacked trail, he was demoralized, because he thought he found some hidden gem.
Change is hard, but once you get the hang of GAIA, ONX will be a distant memory. You can also share way points, etc. I only keep my onx subscription, because some of my buddies are old obstinate fellas that resist change, and this way they can still share waypoints, trails, intel, etc with me.
One other neat feature in GAIA - importing CalTopo Slope Angle Shading. This basically creates a heat map of the topo - I know we can all read topo lines, but this adds another level of interpolation. The red areas are really steep, like cliffs, Black areas are vertical, but then you can see where it changes from orange to yellow it's more passable and it has helped me identify small micro saddles in otherwise really steep terrain.
On our season opener I found such a spot, set up on it, and within an hour I had a 300# bear strolling in. Of course the wind hated my guts that afternoon, but when I went back another time there was another bear, but I bumped him out.
OK, in case you can't tell, I like GAIA GPS. Backcountry it has literally saved my a** more than a few times.