The EP should be connected to the GND pin. The GND pin on the MAX17260 doesn’t need to pass current through the sense resistor, so it may be connected to the cell side. On the MAX17263, the GND pin is connected to the SYS-GND side so the LED current can be measured at startup to determine the LED count. The two kits share similarities in the layout because the MAX17260 (master EVKIT for BC21) was developed earlier.
Connecting GND to Cell – has an advantage of lower offset for devices that are in idle mode longer (no I2C or AP connected for a long time). This is good for primary cell applications, or where raw coulomb counter accuracy is very important. Connecting GND to SYS side has an advantage in chatty applications where the host is always talking. The I2C sink current is not passed through the sense resistor and reduces the raw coulomb counter offset.