I didn't know that Gilfillan made 6 inch grinders but I've also never heard of anyone searching for one either. Knowing that Gilfillan made quality tools, I'd compare it to any other quality manufacturer like Baldor. You can find Baldor grinders for sale online for around 50-75 bucks around here. Note that I'm in an area where vintage tools are plentiful. Your area may not be and prices will reflect that.
Having 1/4 of a picture makes it really tough to give you a value. If you post some decent pictures we can give you a far better assessment.
As for the era, the brass tag tells me it's likely late 30s very early 40s.
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