Are there makers out there that do conical simple system flutes in the key of G?
My Tipple G flute is not conical, but I do consider it to belong in the category of good flutes. Doug is very accommodating and I’m sure would be able to work with you and provide you with an instrument you will enjoy. Sweetheart shows what looks to be a conical G flute on their web site; it’s listed under “Folk Fifes.”
The Sweetheart G flute is good.
Phil Bleazey makes one too.
Also Billy Miller has a G bamboo flute,
cylindrical, of course.
Beyond those mentioned already, I believe Skip Healy makes one in G and I do think I have seen a G flute from Mark Hoza somewhere along the way. I’ve never played a Hoza flute myself. Skip Healy makes good stuff but YMMV. The Sweet folk fifes are a good choice at a decent price. There must be others as well.
Feadoggie
If you’re looking for a keyless conical bore G flute: [u]http://www.sweetheartflute.com/folkfifes.html[/u]
Yes, they’re called Folk Fifes, but they really are conical bore flutes.
Dave Copley makes a G flute, too.
His F flute’s head and foot joint (a wonderful flute) fits a G body he makes for it.
Now I did not know that! There’s not a mention of it on the website either. Thanks, Jim. That’d be the one for me then.
Feadoggie
Of course the way to do that is to get the F flute along with the G body.
I got a delrin G body, in fact. It was a prototype.
I believe (and I think Dave thinks too)
that the G flute sounds less good than the F flute, but it
still sounds better than any other G flute I’ve ever played.
And the F flute is better than very good.
I don’t particularly like flutes in the key of G, having a strong affinity for flutes in the key of F!
But I had a Mark Hoza G flute that was bawdy with a voice like Ethal Merman! 
The Sweetheart G Folk Fife is much tamer.
I have a Sweetheart in F and a Bleazey in G. There seems to be some big jump between the two - the F feels like a small D flute while the G feels like a bigger piccolo/fife. Much more than I would have expected just going by the difference in keys. Both are fun to play, but I must admit the G flute often gets shouldered out in favour of the low G Alba whistle that I also own - don’t really know why?
Chris.
The Bleazey G is a skinny flute. The Sweetheart G is more robust.
My Bleazey, which is a good flute, is nonetheless finicky.
It needs to be blown just right to sound. The Sweetheart
G is less so.
I like G flutes; also A flutes. I have a Sweetheart A that I
enjoy playing a good deal.
I just sent a blackwood F/G corp de rechange (sp?) Copley and Boegli flute to Doc Jones at the Irish Flute Store for consignment. I loved it! but unfortunately have two boys in need of braces, and therefore I’m in need of money. 
He should be listing it sometime soon.