sorry, not sure if you're asking for opinions & suggestions.
if you can layer other clothes enough to make that 40 degree quilt work at 30 degrees (occasionally a little bit colder), then go for it. i personally can't do the Sierras with any bag rated higher that 20 degrees, but i'm a cold-blooded woman. ;)
have you used the gatewood in heavy inclement weather? i just keep thinking about AYCE's article he wrote after using a poncho tarp to do the JMT back in 2003 (it can still be read over at thru-hiker.com). admittedly, that was a heavy-weather year, but on the JMT you never know.
when are you planning on doing it?