You would get better answers to JP-1 questions in the JP1 forums.

Almost any JP1 remote can be upgraded with that txt file to support all the functions of the GTP-500 original remote.

The best source for a JP1 remote and cable is Rob:
[Link: hifi-remote.com]

Usually he is willing to preload an upgrade if you request that at the time you order the remote, so you could get remote and JP1 cable and be able to use the remote for your GTP-500 even before you learn to customize it by JP1. You could even skip buying the cable if Rob loads the upgrade, but I wouldn't advise it. Being able to customize that upgrade (or others) later is well worth the cost of the cable.

To use that .txt file you need EITHER the KM spreadsheet plus Microsoft Excel OR the RM program plus Sun Java (Sun Java is a free download). After reading the txt file with one of those programs, you need IR.exe to integrate the upgrade with an eeprom image for your remote and to transfer it to the remote.

All the software except Excel is available for free download.