Gib can be banana when it is that slender. But the cross section must stay constant. My friend just ordered spare for one milling machine and spent two weekends scraping it.
Can you use the grinder table (bolt it onto milling machine table) and use that as a jig to mill the gib? If you clock the "jig" or table straight you can move the gib, clamp it against the dovetail and mill the top with endmill with reasonable accuracy.
I wonder if I can explain it right: You size the stock to right flatness, then you have a square bit. Then you clamp this square stock into the dovetail using roll pin or whatever means necessary. Now you are offereing it to the end mill at correct angle, because it is in the dovetail in situ. You mill the top straight (it will be proud of the dovetail, because you have unwanted corner at the bottom), then you shim bottom at the dovetail, put this gib strip atop of the shim, clamp, and mill this other side. I may need to resor drawing (suck at it).
I would not use any type of tilting vice on this long/slender piece. You will be fighting a lit to get the setup repeatable.
Pekka