However, once you get used to it, G tunes sound really good in standard GDAE. You've got some low drones, and they act as sympathetic strings in a subtle way even on the E string where the drones are out of reach- the correctly intonated note will sound louder and clearer than one that is "off".
I use GDGD or ADAD or sometimes ADAE. Mostly I play out of standard (GDAE) or tuned down a step from standard, but lately I've been tuned to ADAD and liked it for some tunes I had learned in standard. I'm not real confident transposing to cross-tuning yet, but when it works, it works. I suppose you could play an E string tuned down to a B. It would take a light touch. Not sure what you'd get. Play the tune in whatever key makes it workable in the tuning, or vice-versa. Since most keys and tunings would be absurd, you'd be left with the one or two that actually get used, and with good reason. It's only old time.
There have been a couple of tunes, in my experience, that I originally learned in AEAE, but later found one was really in G (standard GDAE; Old Mother Flanagan. also related to Shippensport) and the other was Hangman's Reel, which I found I could play in GDAE, which in addition allowed playing of one part an octave higher when I felt like it. So in moving an AEAE tune to G, you might find new adventures in trying it in standard GDAE.
GDGB is the equivalent of AEAC# (which happens to be the Canadian tuning for Hanged Man Reel) which is perfectly playable without lightening up your touch much. There are a bunch of fun tunes in this tuning. If you're planning to play the same tunes tuned down, it's hardly a key change and more of a pitch change.
I'm actually fairly new to the fiddle and the tune I was working on was Cumberland gap, I can't really sing in A so I wanted to drop it to A. Also the band I play with occasionally covers wagon wheel in G so I want to learn that.
Just a quick suggestion Louis... cross-tuning your fiddle may be a bit cumbersome if you are playing with a band who likes to play more bluegrassy/newgrassy stuff, since even with geared pegs it takes a bit of effort to retune between each new song in a different key. Some folks do it by keeping two fiddles with them, but that seems cumbersome to me, too! I play Old-Time music, where we all agree to stick to one key for a while, so cross-tuning works great in that environment.
All your A tunes can be played with the exact same fingering in A (AEAE) or in G for (GDGD) or even in F (FCFC) for a super creepy-cool feel! One thing to think about, too, is that cross-A has a bright punchy sound, but cross-G looses quite a bit of volume if you don't have a super bright fiddle. Cross G is my favorite place to be, but It can be hard to hear in a big group, for sure. And cross-F? Well, you'll just want to find a tiny closet with one or two of your closest friends. It's super quiet, but SO fun. You'll be glad you are in a closet!
The two tunings AEAC# and GDGB are called calico tuning (among other things). The most popular tune I know in that tuning is drunkin hiccups AKA Jack of Diamonds. I have jammed (by myself) every old time song I know in this tuning, because you can still play the tune like normal on the lower strings. You get a great sound with the double stop of your raised 2nd finger on the "A"-string with the open "E"-string, and the double stop of open "A"-string and a flattened 2nd finger on the "E"-string. The best song I play in that tuning that is usually meant to be in cross is backstep cindy.
Interestingly, this tuning is also called fanitullen tuning and is (one of) the standard tuning for a hardanger fiddle. Check out the song fanitullen on youtube to hear it in action.