Thanks George. I was re-reading a section of the GTM Configuration Guide and came across the following:
Note: If your installation of the Global Traffic Manager resides on the same system as a Local Traffic Manager, you define only one BIG-IP server. This server entry represents both the Global Traffic Manager and Local Traffic Manager modules.
Unfortunately, this doesn't really clarify anything, as when adding the GTM/LTM the argument could be made either way for adding the system as a single system (single GTM), or as a redundant system (LTM HA pair).
However, if I add the GTM/LTM as a redundant system, then I'm assuming that this will be taken as an implication that the GTM is a HA pair, which may have unexpected consequences if the system were to fail.