When you configure Webex Calling for a location you'd set what type of PSTN service it should use. It can be Local Gateway, ie Bring Your Own PSTN, a Cloud Connected PSTN service through a 3:rd party service provider or Cisco Calling Plan. So the answer to your question is Yes, from a Webex Calling perspective.
However based on how you've worded your question I would assume that your asking about if it's possible to have each site use a specific PSTN service you have in your CM system landscape via the trunk(s) you plan on creating for the LGW setup. If so that's absolutely doable, it's simply a matter of configure the needed call handling in either your Cube(s) for LGW or in downstream CM where the Cube(s) used for LGW(s) connects with.