No that is incorrect. The older databus is <b>not </b>GMLAN. Its not even any sort of CAN.
Its J1850 variable-pulse-width and called "Class 2" bus. It is in no way compatible with the CANbus triple, or any CAN-related hardware. You need a specific transceiver chip that is designed for J1850 VPW. The ELM327 will work fine....they're about $15 on ebay.
The GM trucks/SUV's went to CAN (high speed GMLAN, 2-wire) in 2006 on the Duramax diesel trucks only. And that CAN was ONLY FOR POWERTRAIN. The rest of the body/chassis/4wd/ABS was still the old Class 2 bus. GMLAN on the 2006-2007 Duramax trucks was only for ECM-TCM-GPCM (glow plug control module) communication, and for scan tool communication. The ECM acted as the gateway between GMLAN and Class 2, because the instrument cluster and all that only speaks Class 2.
The 2006-2007 non-Duramax pickup trucks were still Class 2 only...no CAN/GMLAN anywhere. (unless it was one of those rare 04-06 hybrid trucks, those had CAN/GMLAN, but thats beside the point).
With the redesign of the GM trucks and SUV's in 2007.5 (for the trucks, SUV's got redesign in 2007), they went "full GMLAN" and no more Class 2 was used. On 2007/2007.5-2013/14 GMT-900, low speed GMLAN (33.3k single-wire CAN) was used for body/interior, and 2-wire high-speed GMLAN/CAN was used for powertrain/chassis.
<b>So you CANNOT just make a blanket statement like "GM went to CAN/GMLAN in xxxx year"</b>......because there was a large transitional/crossover period. Some cars went full GMLAN and eliminated Class 2 completely as early as 2004. Some GMLAN vehicles were "partial" GMLAN (CAN for powertrain only, but kept Class 2 for body/interior) as late as 2012!! The Colorado pickup truck I think was the last GM vehicle to use Class 2 for body/chassis....but the Colorado went GMLAN/CAN for powertrain in 2007 or 2008 I think........because OBD/EPA standards dictated that all vehicles had to be CAN (for engine/powertrain, EPA doesnt care what car mfg's use for body/interior/chassis) for the 2008 model year.
Clear as mud?