The GM warranty guide does include the injection system as covered components under the diesel engine warranty:
Diesel Fuel Metering System: injection pump, nozzles, high pressure lines, and high pressure sealing devices
In addition, the injection system is also listed as a covered component under the extended emissions performance warranty.
The warranty guide comes with every vehicle. Sometimes highlighting the applicable covered components, and showing the service writer, will help them properly code the components so they show up as covered in their system.
If they are denying coverage due to damage from contaminated fuel, then you can file a comprehensive coverage insurance claim.
