Hmm, GQL seems to be somewhat similar to what I implemented in SuperTool (isotammi-addons/source/SuperTool at master · Taapeli/isotammi-addons · GitHub).
I think SuperTool already has many of the features discussed:
- a simple query syntax
- an UI with input fields and a way to display the results (user can specify which expressions are displayed)
- SuperTool can create a custom filter using an arbitrarily complex filtering expression
- SuperTool can use any Gramps internal methods and the user can define auxiliary functions as needed