I believe gmtd scantool only does OBD1. You can use carcode or a tech 2 or several others. Be aware that snap on mt2500 doesn't read correctly. Maybe ferm can chime in on the subscription for the tech2. I don't think it needs one though
I have been using Autoenginuity for years and like it fine great for doing multiple makes and latest version can be run off a Tablet or Ipad. The customer service is excellent also. Yes it is dollars to get the Bidirectional GM package but once you own it your done. The trick is if all you are doing is servicing your 6.5 not to update it I stayed on a 2011 version for years. Your subscription to the GM package will expire after two years but it will still keep working so long as you don't update the program. As to having to pay extra for other make Bidirectional packages any Scan Tool will charge you the pleasure for being all encompassing.
Whats also neat about AE is that you can graph and record multiple sensors whilst driving to help diagnose.
An example of their customer service, one year I paid for the upgrades thinking I'd finally get access to the ABS bleed procedure. Turns out they could not add it for the 97 and under because funnily enough it is a mish mash of OBD2 and some OBD1. When I discovered this they with no complaints refunded my money and helped me roll back to the older version, they could of just said sorry and kept their money.
I forgot to post a update ive been driving it no issues at all my matco maximus 2.0 says my actual injection timing is 12 not sure if thats to high or not a guy from a diesel shop said something about 10 to 15 is fine? Anyway i have no 'spark knock' or black smoke only very very little puff of cold start that last maybe 2 seconds. Ive seen where some people are at 10.9-11? I would appreciate your guys input thanks. Ps i also upgraded my water pump and got rid of the zcp097 Chinese injectors for bosch and installed my boost gauge and the diamond eye cross over and for those wondering about the super 54 its great and has a nice sound when she is nice and warm idk about cold i let her warm up before driving it. Also thank you guys