This page ( https://www.allen-heath.com/key-series/gld-series/gld-fx-processing/ ) covers most of the GLD’s effects options, with the exception of the Dimension Chorus which was added in a later firmware update. You can click on any of the images on this page, to hear samples of the different effects.
The FX sends are basically like Aux sends except they’re routed to one of the 8 FX slots. The FX returns are like input channels that return the FX to the mix. They don’t have all of the same functionality as a regular input channel, but they are still basically channels that don’t eat into the channel count of the 48 input channels (they are in addition to).
The FX slots or RackFX are the 8 virtual locations that you can populate with any of the effects in the link above. This would be equivalent to an actual physical FX rack in an analog setup, and you can setup any effect from the available selections into each of those 8 slots.
Additionally you have the option to “insert” any of the RackFX into a single input channel, or output bus. This can be done from the “rear” of the specific FX unit, or from the channel’s “insert” tab.
.
