This would appear to relate to earlier "blocks" used in the Irish Register. Looking at the last two letters, S- has been used from SL onwards in the EI-ES block, so Ryanair are using EI-GSA-GSK and stopping there (at K). As for a "last two" beginning with X, XA-XF were used in the EXA-EXF series, hence the GX- series begins with GXG. Hope this logic makes sense!