Hi Daxer,
There currently is no security/parameter in Global Address Book that will allow you to secure the information per legal entity. Both AX 2012 GAB parameter is not enabled in D365.
The only way to restrict by legal entity in D365 at this stage is through customisation.
The Parameter available in the standard D365 is the one below:
Private location security
More information on this can be found through the link below;
docs.microsoft.com/.../configure-global-address-book
I checked with our team as well and there was a private fix provided for some customers to add the feature through a private fix.
This however may impact performance.
For more information on the private fix, please log a support ticket through LCS link below:
https://lcs.dynamics.com/v2
Reference this LCS issue link:

fix.lcs.dynamics.com/.../Details
This fix is already included in D365 8 and above.
NOTE:
The fix does not add the security features as advised in the LCS link above but instead helps to ensure that the application includes the required directory model file changes for GAB security to enable secure by legal entity and/or secure by address book via a customisation/private fix.
Regards,
Marian