It seems that the GUIDRV_FlexColor driver is already compatible to this controller. We have two GUIDRV_FlexColor variants which are mostlikely compatible. These variant are GUIDRV_FLEXCOLOR_F66720 and GUIDRV_FLEXCOLOR_F66709. I would try the GUIDRV_FLEXCOLOR_F66709 first, this one supports already a lot of controllers.
The only thing I'm wondering about is that the data sheet of the GC9A01A does not say anything about reading the display data RAM (DDRAM). If it is not possible to read out the display memory the GUIDRV_FlexColor driver requires a cache.
Of yourse, if you are using an RGB interface for the controller you can use the GUIDRV_Lin driver. But this would require a second display controller which generates the proper display signals (most like a controller integrated in the MCU).