The channel number selects the approach, since one GBAS station supports numerous precision approaches within a certain range (iirc up to 49 approaches within 23 NM, or was it the other way round?) there needs to be a method to select the approach in question. In EDDW it is only two approaches, one for each landing direction.
Anyway, i would love to get a reference for Avenger's information, as i said i suspect that you need a FMC for it, but i cannot find it in our documentation although we are certified for full operational use of GLS as one of very few airlines in europe and currently fly CAT III evaluation trials for it.