What do the letters GL SLK ZX refer to after the brand name?

Most of these letters that follow the brand name refer to the different packages that where available at the time the car was made. Some refer to Sports packages or high performance models others might refer to one that has all the available features such as power Windows seats and so forth. Hope This Helps, EzForJesus

AnswerI believe they are RPO codes (Regular Production Options).

Some cars have taken on nicknames of sorts due to this. An example is people referring to Camaro and Firebirds as having LS1's, which is referring to the engine.

Sometimes manufacturers use them for marketing. WS6, Z06, Z28, etc. (As you can see, I'm f/y-body biased, hah!) as package option codes.

gl means grand lux mg - Morris garages gt-grand tourer this was on most older cars

Mercedes - SLK. The S stands for Sport (sports car), L stands for Light (light weight) and K stands for Kompressor (supercharger). All MB's with superchargers will have the Kompressor logo on them somewhere. Either on the side or rear.