Chipping a turbo diesel can be fun and add some power, but it comes at a price just like anything else. I race TDI cars and have for 6 years. My Jetta made 225 HP and 399ft tq, and still got 40+mpg cruising. I spent a lot of money to get that and I constantly working on it.
A gas car, no turbo, you can get a few HP and thats really it. You might move some power around, but no way to get big gains on a normally aspirated engine. It is already tuned for max economy and power from the factory.
For example, a Honda civic engine produces the same power as a Ferrari V12 per cylinder. No secrets or science there. Limited by technology.
Never buy chips off ebay. Go to a reputable tuner.