This is a hack on Statamic's Suggest fieldtype to allow options to be passed in via a separate file. This way global variables can populate the options.
Paste the code from ft.suggest.hack.php
into cp/fieldtypes/suggest/ft.suggest.php
after the "Hardcoded list of options" section.
In your fieldset, do something like this:
brand:
display: Brand
type: suggest
file:
path: _config/global_variables.yaml
variable: brands