Without a turbo there is only minimal power gains you can get from computer changes. Diesel engines almost all have a turbo, gas engines almost never to. In addition some of the emissions stuff on diesels does cut efficiency by a significant amount, there isn't much gain in a gas engine if you are willing to ignore emissions.

Don't confuse the above with zero gain from custom tuning a gas engine, just that the gains are not nearly as great as you can get with a diesel. If you are running a gas car on a track you can get measurable results from custom tuning - but it isn't nearly as noticeable in day to day driving.