I'm not sure if you can post ACTUAL GPS coordinates for the message, however
You can specify a location object for the message. You can read here for more information on creating a message and what options you can go through.
The location parameter is called place :
an object containing the identifier and name of the page associated with this location, and a location field containing geographic information, such as latitude, longitude, country, and other fields (fields will vary depending on geography and availability of information)
It looks like you can after latitude and longitude ...
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