I believe the gutter is in the left margin where the break-point markers and the program pointer show up.
That's right, this is how the word is generally used in text editors. Another meaning for gutter is the "inner" margin of a page in a book (i.e. the right margin on a left side and the left margin on a right side).
Scintilla calls that line "edge", I would call it "silly line at 80 characters", but neither of these is really a good naming
I am not even sure what it is good for, formatting for a fixed printer width? (the last time I printed out sources was around 1985, when thermal printers and 9 pin dotwriters were still considered state of the art, so I wouldn't know
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"Right margin" as you said might still be the best choice, even though "margin" suggests that it breaks lines going beyond (which is not the case).