The Gnomic projection is defined by the word GN and the following items
the origin of latitude in decimal degrees
the central meridian in decimal degrees
the false easting in metres
the false northing in metres
Here is an example of a Gnomic projection coordinate system.
TEST_GN "Test gnomic" REF_FRAME NZGD49
PROJECTION GN -45 170 300000 700000