Doug,
This may not be exactly a full size plan for G scale, but there is enough detail here to freelance one of your own from these plans. http://on30annual.com/scratchbuild-a-gallows-turntable/
Not sure what scale you are working in, the drawings are in O scale (1:48). To go to other garden scales, blow the drawings up by the listed percentage:
1:32 = 150%
1:29 = 165%
1:24 = 200%
1:22.5 - 213%
1:20.3 = 236%
And for those in the ‘Maine 2 Footer’ scale, 1:13.7 = 350%.
I also ran across a thread in another forum referencing Narrow Gauge and Short Line Gazette, Nov/Dec 1989 and Jan/Feb 1990 issues, in S scale (1:64) by Al Armitage.
1:32 = 200%
1:29 = 227%
1:24 - 266%
1:22.5 = 284%
1:20.3 = 315%
1:13.7 = 467%
Note I have deliberately dropped the decimal on the percentage as most copy shops you might take the drawings to will give you a ‘deer in the headlights’ look if you start giving them anything other than whole numbers.
And while digging for some kind of drawings, I ran across this forum post with some pretty detailed build information on a gallows turn table. It is for what looks like HO but the techniques might apply to, or be workable for G scale. http://modelersforum.com/scratch-building/building-a-gallows-turntable-for-the-horace-and-william-creek-r-r/