@George Williams provides the key element to understand why unique channel numbers are used for procedures not requiring individual radio channels, this is why I selected his answer. I'm only providing complementary information I could find based on George's explanation.
The GNSS 5-digit channel number is a way to select an approach on avionics equipements.
Gables G7501-01 panel (dual channel) used on Boeing 737. Source
In addition of tuning the receiver, this can also recall the associated approach path from the navigation database. Because the channel number can be used to select an approach path, it must be uniquely defined.
It can also be related to radio frequency but hasn't to, so "channel" is actually misleading. The radio part doesn't come from the GNSS signal frequency which is well known by itself, but from the GNSS augmentation system. GNSS position is usually improved (augmented) by GNSS signal correction to reduce the delay introduced by atmospheric refraction.
There are multiple ways to evaluate the required correction, and to broadcast it to the users. Broadcasting encompasses using ground-based transmitters (e.g. DGPS, known in the US as LAAS, and by ICAO as GBAS) and satellite-based transmitters (e.g. WAAS and EGNOS, collectively known as SBAS).
GBAS corrections are essentially for users in a small area around the evaluation station, and different VHF frequencies must be used to not interfere. Transmissions are known as VHF Data Broadcast (VDB).
The channel number is computed from the frequency used, and also from an index identifying the system to be accessed, either the reference path data selector (RPDS) or the reference station data selector (RSDS), which is usually 0. The channel number is determined using the formula: $\small 20,000 + 40 (f – 108.0) + 411s$, $\small f$ being the VDB frequency and $\small s$ being the index.
GBAS approaches have channel numbers between 20,001 and 39,999.
SBAS corrections are related to a wide region, e.g. a sub-continent, transmitters are based on geo-synchronous satellites and broadcast on the same L1 frequency than the GNSS constellation. So their frequency is well-known and no channel is to be selected.
The key information, provided by @George Williams, is the channel number in GBAS procedures allowed avionics manufacturers to select the approach path in the FMS and the VFB frequency in the NAV receiver. SBAS channel numbers have been reused as selectors for the approach path.
Channels 40,000 to 99,999 are reserved for SBAS approaches. Numbers are allocated by ICAO. On the past FAA used to assign numbers for WAAS area and Eurocontrol for EGNOS area.
For GNSS approaches, the channel number is equivalent to the procedure ID (here W36A, W for WAAS). With two identifiers for the procedure, it is possible to confirm the correct procedure has been loaded into the FMS. It's equivalent to tuning an ILS and checking the morse identifier. Navigation systems are required to allow selection by either of the identifiers (source).