I have dabbled with the same problem with AgoRapide which offers queries through GET (although on a much simpler level than PQL / GraphQL).

I just present the query in readable form together with the output, letting the user choose between copy / paste the URL or copy / paste the readable form.

Example:

http://arnorthwind.agorapide.com/RQ/dt/Order/WHERE%20Shipped...

will present the alternative form

   Order

   WHERE ShippedDate = Today

   SELECT OrderId, ShippedDate, CustomerId, Customer.CompanyName, OrderDetail.Sum.Sum() AS SumOrder

together with options for choosing Yesterday, 2 days ago and so on.

See more examples at http://arnorthwind.agorapide.com/