Compiled here is a list of cars that ride on a certain GM platform, and the layout of the platform. Good reference in the future. Only doing current production vehicles in the USA or else this would be a cluster fuck.

Alpha: Front-engined, RWD/AWD, Short and Long Wheelbase

SWB:

Cadillac ATS

LWB:

Cadillac CTS

Unknown:

Chevrolet Camaro (2016+)

Cadillac CT6 (?)

Gamma II: Front-engined, FWD/AWD

Chevy Sonic

Chevy Spark

Chevy Trax

Buick Encore

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Delta II: Front-engined, FWD

Chevy Cruze

Buick Verano

Chevy Volt

Cadillac ELR

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Epsilon II: Front-engined, FWD/AWD, Short and Long Wheelbase

SWB:

Chevy Malibu

Buick Regal

LWB:

Chevy Impala

Buick LaCrosse

Cadillac XTS

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Zeta: Front-engined, RWD, Short and Long Wheelbase

SWB:

Chevy Camaro

Chevy SS

LWB:

Chevy Caprice (Municipal use only)

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Y-Body: Front-engined, RWD

Chevy Corvette

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Theta/Theta Premium: Front-engined, FWD/AWD

Regular:

Chevy Equinox

Chevy Captiva Sport (Fleet Only)

GMC Terrain

Premium:

Cadillac SRX

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Lambda: Front-engined, FWD/AWD

Chevy Traverse

GMC Acadia

Buick Enclave

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GMT 31xx: Front-engined, RWD/4WD

Chevy Colorado

GMC Canyon

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GMT K2xx: Front-engined, RWD/4WD/AWD

Chevy Silverado/Silverado HD

Chevy Tahoe

Chevy Suburban

GMC Sierra/Sierra HD

GMC Yukon/Yukon XL

Cadillac Escalade/Escalade EXT

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GMT 610: Front-engined, RWD/AWD

Chevy Express

GMC Savana

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Nissan B-Platform: Front-engined, FWD

Chevy City Express

There, I have every platform GM uses in the USA and the cars that grace them. Hopefully this is helpful to someone at some point