okay, i didn't have high hopes for those, but they're easy to at least try.
slightly more complicated, and the only other idea i have:
open GB
create a new project
once it's saved, quit GB
control-click this newly saved GB project in Finder, and choose Show Contents
throw out everything that opens in that window
leave it open
control click one of your bad projects, and again "show contents"
select everything in the bad project's window
hold down the optionKey and drag all those bits into the New project window we left open above.
close these two window we've opened via the Show Contents bit, and try opening that new project we've created (double-click the new file, not the old one).
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i've never tried this before (never had a "windows damaged" file with which to play), so odds are probably 60:40 in favour, but it's about the only Hail Mary thing i can think of that might work (and maybe it's more like 70:30, really).
good luck! ~~Hang