Chemical chrome stripping?

Chroming over engraving does not come out nice. I worked in a chrome plating shop (Until I started coughing up blood and a co- worker said,"You get used to it"....) for a short while, and chromed some personal things with engraving. It fills in the engraving unless done very thin. I'm not sure how to strip chrome, but unless your friend wants an engraved aluminum gas cap that was previously chromed, I'd suggest not doing it. Perhaps there have been advances in chrome plating in the 30 years since I worked there, call around your local area's chrome shops.

Chrome on aluminum is usually done over a copper wash I think....that might be nickle tho......anyways removing it would likely cost more than a new, raw billet gas cap.
I would suggest starting with a raw cap, engrave, and giving it a high polish (which is shinnier than most chrome). If he wants it weather resistant finished it can be clear anodized, which will cause some polish loss but will still be shiney, and your engraving will show just fine.

I've had some interesting fights with paintball guns and nickle/chrome finishes.....

Often both copper and nickel.

I had a chromed Luger stripped once. I did a lot of research on the subject beforehand and they usually use both nickel and copper underlayment. Stripping is NOT something I would do myself unless I could properly manage the waste product (whether sanding, blasting, or chemical strip). It's pretty nasty stuff that you don't want to dump down the drain. Find a local shop near you. Should be cheap to strip it.