Hetty, I don't know orchids but proper use of grex in orchid names are talked about briefly here.
http://apps.rhs.org.uk/rhsplan...
"...In some plant groups, notably within orchids, where complex hybrid parentages are carefully recorded, the group system is further refined. Each hybrid is given a grex name (Latin for flock) which covers all offspring from that particular cross, however different they may be from one another. Individual cultivars may then be named and propagated by division or micropropagation. Although a grex is similar to a botanical hybrid in principal, backcrossing a member of a grex with one of its parents results in a new grex, with a new name, whereas backcrossing a hybrid makes no difference to the hybrid name. In constrast to groups, with grex names no brackets are used and grex is abbreviated to g. - for example, Pleione Shantung g. is a popular grex of hardy ground-living orchids while P. Shantung g. ‘Muriel Harberd’ is a particularly good cultivar, selected from the grex. ..."
Elsewhere on the RHS site they indicate grex is abbreviated gx.
