These instructions demonstrate the process for EPSG:3572.
Geoserver Admin > Tile Caching > Gridsets > New Set Name / Projection to EPSG:3572, when you tab out of the Projection box it should autocomplete a bit of information here, such as Units, Meters per Unit.
Under “Gridset Bounds,” > Compute from maximum extent of CRS. We’ll need these numbers to configure Leaflet. (An error at the top may show, “Field ‘Gridset Bounds’ is required,” ignore that).