Amstrad/Locomotive provided DDMODE0.PRL, DDMODE1.PRL and DDMODE2.PRL which implement drivers for each of the CPC's screen modes.
ASSIGN.SYS is a text file which must exist along side gsx.sys and which contains the mapping of device id to the driver that implements that GSX device. When you program GSX you open a device by it's its id.
Example content of ASSIGN.SYS