You can customize your dashboard with events. The thread below shows when I learned to make these events. You’ll have to either find an adapter that handles the GL codes, or make a BAQ that searches for them and run a dynamic query to return a list (or no) to validate the text.
Alternatively, you could make the grid field a dropdown instead of a text field, which would automatically validate because they have to pick from the list.
All of the this will take at least some coding (maybe not the dropdown 
) , but it’s not incredibly difficult. You will need to deploy your dashboard as an aassembly and customize though.
edit
Also, relating to your first question about the context menu, does a search function show up in that list? That’s how you would return the value.