Well i also got some sambapos4 and there is no GQL support, and i am not so familiar with GQL i see that @QMcKay posted something in here GQL Modules - GraphQL Modules for Remote Client Browsers but it seems to be tested with version 5.1.61 ( i tried testing with 5.1.60 but the auth is not working ) and i do not have that as i think thats for devs only.
I’ve manage to test the GQL from the local browser using localhost:9000 and its working really nice, but i dont seem to find a way to inner join some sql tables. I think i can do that by creating a sql report and grab that using GQL but still have to read on how to do that as i dont know yet 
Using PHP and MSSQL queries will be much more simple for me and also for other users and this also can work with lower versions of Sambapos as not all users use the latest version.