http://en.wikipedia.org/wiki/G

From ancient Roman cursive. See, for example, discussion at bottom right and following page of:
http://archive.org/stream/romancursivewri00hoesgoog#page/n281/mode/2up

For various examples in more formal medieval writing, take a look at:
http://medievalwriting.50megs.com/scripts/examples/halfuncial.htm
http://medievalwriting.50megs.com/scripts/examples/visigoth.htm
http://medievalwriting.50megs.com/scripts/examples/merov1.htm
http://medievalwriting.50megs.com/scripts/examples/carol5.htm
http://medievalwriting.50megs.com/scripts/examples/carol4.htm

Reading the Wikipedia article suggests that the single-story lowercase G is not merely a feature of modern sans-serif typefaces, modelled on children's handprinting, but was also found in blackletter.

And, indeed, that is the case.

Thus, one might better ask: where did the double-storey lowercase G come from? The Wikipedia article also gives an answer to that: in Roman type, it allowed descenders to be shortened.

Which doesn't quite explain why it was found in the typefaces of Nicholas Jenson, instead of waiting until American Type Founders came along...

I like 'closed tail' and 'open tail' a lot more then 'single story' and 'double story.' The later just seems so modern. I would find it a little hard to believe that Garamond referred to the forms as such (though I don't think her ever produced the open tail variety, am i mistaken?).

Except you can have a binocular "g" with an open tail...
In fact that's my favored form.

hhp

you can have a binocular "g" with an open tail

Very open, at times:
http://typophile.com/search/google/danish%20g

As well as a closed tail on a single story. Maybe it's best to just smile and nod.

A binocular "g" with an open tail is what's known as a "double story with walk-out basement".

I like double story with walk-out basement. I seem to recall a discussion somewhere here on names for punctuation points and such like. When I was an apprentice comp., way back when, we used to call exclamation marks "dogs' willies".

We need to know when the first appearance in type was, for the single-storey g in:

• Serifed italic
• Serifed roman
• Sans italic
• Sans roman

Does that exclude Granjon's Civilité?