There is a table headed GPIO usage on E-Paper HAT in the section 'About this project' which lists all the pins used and what they are used for. Please note that, although the real time clock uses the i2c bus, you can still hang other devices on the same bus, as long as they don't try to use the bus address 0x68 which is reserved for the clock chip. I have tried this and it works fine. Also, although data for the display is sent over the SPI bus, I understand it is still possible to connect one other SPI device using the same MISO (GPIO9 or P21), MOSI (GPIO10 or P19) and SCLK (GPIO11 or P23), as long as you use CE1 (GPIO7 or P26) as the select line. I have not tried this. There is a distinct possibility that three more pins will be used for an additional (secret :-) feature, but I have tried as a matter of design, to leave TXD (GPIO14 or P8) and RXD (GPIO15 or P10) free in case anyone wants to hang a serial console on there and also CE1 (GPIO7 or P26) for the second SPI channel. I hope that covers most people's questions about the GPIO. If I missed something, please shout out and I'll try to provide an answer.