Checking out that first plugin link, I'm not sure what the author is talking about - I use grids in GIMP all the time, and any changes I make show up immediately
Perhaps they are using an older version? Or the author is chnaging the default grid under options, instead of the active one? I'm not sure. But it's always worked fine for me.
In any case, still awesome. I never even thought of mapping interiors by using snap-to-grid! How did I never see that? >3>;