The GBCM was introduced on trucks in 2005 as a way to increase fuel economy by not having the alternator charging at full potential all the time. Prior to that, there were just two wires going from the alternator to the PCM, and the PCM controlled the charging. I have in the past had a 2006 swap programmed like 2004 and wired it as a 2004 also and it worked perfectly, although that did require a 2004 TAC module too since they have to be matched to the tune. However, it has since been figured out that you can just leave the GBCM on the battery cable and wire it accordingly, but without the wire going to the BCM and it will work. Here's a thread on the topic.
https://ls1tech.com/forums/conversio...le-gbcm-2.html