Hello & Welcome,
May I ask what scan tool you are using?
Do you have a plug-in module that is capable of reading the GM ABS system? Most scan tools do not have the capability to do this without an extra module.
I am not really familiar with that particular brand of scanner myself.
Unless you have low brake fluid, what usually happens is one of the wheel speed sensors, or the wiring to them, has trouble.
I have a description of the system below (as a PDF file) and the schematics for the system.
You could check the wiring harnesses going to the sensors and see if there is anything obvious, like pulled wires or dirty connections.
Please let me know what you find or if you need more information.