Not even opening the GMS would be perfect, but I have to run the game constantly as I implement D=
About string completion... one of the weirdest things about GMS1 is it doesn't even compare variable names. A brief oversight and you misspell, getting a bug that can take some time to figure out why (unless you're lucky it's on a "if").
In some situations it is feasible to work with "labels" instead of values in variables, eg: The state of a menu with many submenus is more "readable" with something like menuState="graphicOptions" than menuState=7.
As design complexity increases, having this type of string autocomplete would be as beneficial as comparing variable names. But I fully understand that this can be difficult to do (everything else that this tool offers is already very very nice).