Yeay, I got it working! Thanks for pushing me in the right direction. Turns out there is no default SS pin on this board and I had everything else wrong too :).
Being a newbie, I did have a hard mapping the commented constants with the pins on the board to the pins on the display and with the calls for the class. In case it helps anyone else:
GxIO_Class io(SPI, A, B, C);
A="slave select" = CS on the display -> I chose 21 on ESP (there is no default SS pin on this board!)
B=" DC " = DC on the display ->I chose 23 on ESP
C= "Reset" = RST on the display ->I chose 22 on ESP
GxEPD_Class display(io, D, E);
D="Reset" = RST on the display ->I chose 22on ESP
E= "Busy" = BUSY on the display ->I chose 17 on ESP
Furthermore,
CLK on the display should be connected to SCK on the ESP
DIN on the display should be connected to MOSI on the ESP
Okay, off to do some real testing now 