The ESX-8GT is an expansion module for the ES-4, ES-40 or ES-5, adding 8 on/off outputs for use as gates, triggers, clocks etc. from one expansion header.
The ES-4/5 Controller software (part of Expert Sleepers Silent Way) also allows the ESX-8GT outputs to be used for DIN sync or MIDI output* (note - not just MIDI clock output, but as an actual MIDI port, capable of sending any MIDI messages). Since these outputs are driven from an audio connection, they have sample-accurate timing, which no USB MIDI interface can claim. See the accessories page for the necessary cables. Note there is also the ESX-8MD module, which is dedicated to MIDI and DINsync duties.
Power is supplied by the hosting ES-4/5/40 module, and is used for the LEDs only. I.e. with no LEDs lit, the current draw is 0mA.
The front panel is drilled for both Doepfer and Analogue Systems mounting holes (using a 'slot' rather than a single round hole).
Functionally equivalent to the ESX-8GT mk2, the mk3 adds illuminated jack sockets, and matches the look of the ES-8 and the other Feb'17 updates.