There are two different versions of “ALDL” that GM used. 160 baud, and 8192 baud.
Both of your GMC’s are TBI…the '89 is definitely 160-baud. Not sure about the 1995. 1996 would have gone to Vortec/J1850.
They are both somewhat different. 160-baud came first obviously, and was uni-directional. The ECM could only output info to a scan tool. Obviously the refresh rate was painfully slow, and there was only about 25 PIDs, but considering they introduced it in the early 1980’s, it was impressive for the times.
8192 is bi-directional, but as far as I know, its still request-driven and there are some annoying/complex master-slave relations to work through…so its a little trickier to work with and reverse engineer than the later J1850 and CAN…
Plenty of info out there about it though, if you google it.
Im sure two of the GPI/O’s on the M2 could work great with 160baud and 8192 baud…but i doubt the Macchina team would want to put much effort into writing software for it (understandably)…so you’d be on your own writing an Arduion program to interface.