You can fully automate Gain, but you have to do so outside an Automation Lane. In either a Track or Take Lane, select Clip Gain, and you should see a Red envelope. That can be edited with the pencil tool, or, like I do, with precision dots. Unfortunately, I don't think you can automate Gain by the knob, which I don't care to.
I prefer Gain automation to Volume Automation, simply because I can get this right before beginning the mixing process, and never, EVER have to worry about automating volume afterward. If I need to bring up the level of a guitar for an entire mix, I just grab the fader and change it. Never needed Automation lanes for that.
Beware, though, that there are some very easily found cosmetic bugs in X2a for Clip Gain views in Take Lanes. You may have to select between Clips and Clip Gain at times when the Take Lane doesn't display what the View calls for (i.e. Still see the Gain envelope when you shouldn't).