I recently installed one of the upgrade kits to an ugradeable GG1. The kit includes electro couplers, new brackets to mount them to the leading and trailing trucks, speaker, TMCC and Railsounds electronics, wire harnesses, new lights, and a new leading truck wheel set with magnet for a speed sensor. The kit was easy to install and works well. It does not include cruise control so the operation is not as good as modern systems but it is adequate.
I bought this kit at a local show and the cost was quite reasonable, much less than ERR and Railsounds kits. However, as I said it is not as capable as current stuff either.
Rolland