Who makes a 'chip' for the G20?

[G-Forces]

General
You cannot get 'chip' upgrades for any year G20. You can however get an ECU upgrade. An upgraded ECU is only really needed if you add a turbo, swap a DET motor or for any normally asperated mods past mild cams (ie, race cams, high compression, stroker kit). Aside from raising the rev limiter an ECU in a stock car will give minimal performance benifits.

Who makes them?

• Jim Wolf Technology (JWT) is probably the best ECU upgrader for the G20. However ECU's are only available for the 1991 to 1996 models. The 1999 G20 can use a 1996 ECU with some rewiring. The modifications are minor. Using the 1996 ECU with 2000+ models are unconfirmed.
  
  JWT can program the ECU with a number or different programs depending on the mods you have. Once you have the JWT ECU it can be reprogrammed by JWT for a fraction of the price. You need to either send your current ECU to JWT for tuning. Or they will charge you a 'core' and when you receive they JWT ECU and install it you send you old ECU back. JWT will then refund your 'core' charge.
  
• JUN can also modify your ECU for any application. However it's very expensive and you need to send your ECU to Japan for an undisclosed amount of time. On the other hand JUN is one of the premier tuners in Japan and the results should be excellent.
  
• Unichip also makes a promising product. This is a piggy-back system that interscepts the signal to and from your ECU. The system needs to be tuned on a dyno by a Unichip certified technican. There is not enough information to determine the effectiveness of this mod. It would most likely depend on the experience of the fellow tuing the unit.

Any other ECU tuners that have an application for the G20 are largely untested or unproven. Use at your own risk. If any ECU tuner's products are proven to be effective they will be added to the list above.