Possible to program ECM to the Cadillacs with vxdiag vcx nano gm gds2?

Yes, you can follow the replacement guide here.
This is done thru SPS. It should be the same as new Techline Connect.

Model example:
2005 Cadillac CTS 3.6L

Replace and Program Cadillac ECM Module:

In total it only took 30 mins.

Step 1: Make sure you disconnect the negative battery cable before starting, the bolt size is 8mm/5/16″

Step 2: Next, remove the plastic cover and you'll see the ECM towards the bottom.(Circled in picture below)

Step 3: There's a tab (Blue arrows) in the center of the connectors that you'll need to press while lifting the metal clamps. The top one was broken on mine so I just used a flathead screwdriver to move the tabs (they're sticking out at the top, circled on red) down while pulling on the connector.

Step 4: Now all that's left is to remove the bolts, the 4 on the corners (Circled in blue) are 10mm/3/8″, while the black ground wire (Circled in black) is being held by a T20 bit.

Step 5: Now just reinstall the bolts(do them first) and connectors (push the clamp/the top tab down while holding it in place) to your new ECM, and you're done being under the hood. Protip: Make sure the ground wire is facing up before screwing it in, I made that mistake and had to take it back off and turn it the right way.

New ECM installed:

Step 6: Now here's the fun part, if your ECM is used or brand new it will have to be programmed. I bought mine from eBay that programs them to your VIN before shipping, if yours is used it can driven to a shop after performing a 30-minute key relearn. If you got one that's already programmed like me just do the relearn procedure and you're good to go.

You can also program them yourself with a device called the VXDIAG VCX Nano GM (The tool is ~$119 and the fee to access SPS is $40).

Step 7: Now just follow these steps to perform a Idle Relearn:

1. Turn OFF the ignition for 30 seconds.
2. Turn ON the ignition, with the engine OFF for 60 seconds.
3. Turn OFF the ignition.
4. Turn ON the ignition, with the engine OFF.

Step 8: Congrats, you're all done!

If you do end up using the Nano, it can do the key relearn in 10 minutes with this option: