Since API 39.0 it is possible to access GlobalValueSets through the Tooling API. It is possible to access the GlobalValueSet without adding the field to an object.
The following REST call should be used:
GET /services/data/v39.0/tooling/sobjects/GlobalValueSet/{ID}
Where the ID can be found in SalesForce > Global Value Set > The Global Value Set you wish to access via the API > Grab the ID from the URL.
Hope this helps!
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