GLS approaches are identified by a "Frequency" and a normal identifier. However that "Frequency" is not a normal nav frequency so you have to be able to enter digits that are not normally used in nav frequencies. In our NGs we have to switch the nav mode of the nav radio to GLS and then enter the GLS-Frequency that way.
Nice example can be seen on the trial GLS in EDDW, for example on
runway 09 right above the map picture (not to be used for actual navigation).