All these are good ideas, but I do an extra step as well. Get a bunch of those composition books like we used in high school that have lined paper. I write out all the specifics for the gun and any info such as where it was purchased, when, from who, for how much, etc.
Next, I'll flip a few pages, and when I shoot it I'll put the date, where, with who, the round count and any other important info such as malfunctions or amazing shots made. Anytime I add an accessory or have work done I also record the date and what was done.
IMO, having these composition books for each gun helps with maintaining them and tracking value increases. It's like a diary for each gun. I also feel that if I ever sell one, it could increase the value. In addition, years from now after you pass away, your family will have an invaluable resource regarding your collection.
I also keep a comp book for all of my training just in case.