First, you'll have to get a set of special strings for the G tuning. Not a lot of companies sell them, but Southcoast might. .
Aquila has a set, exactly for this purpose, see here:
http://www.aquilacorde.com/produzione-monofilamenti-musicali-bulks/173/new-nylgut/?lang=en
You have to scroll down to the TENOR section and you will see a DGBE set #11U.
Second, the lower tension will mean less energy from the strings - it won't be as loud or have the full range of harmonic overtones.
I can confirm this somewhat. I have been testing string sets on my recently acquired tenor Flea, and in order to get more sustain, had been evaluating tuning down a minor third, re-entrant, to E-A-C#-F#. I tried this with both Worth Brown (BL), and Oasis Bright High G sets. While I did get more sustain, and a sweeter tone, the volume was about 20% lower than with the same strings at GCEA tuning. I figure the main reason for the reduced volume is that the strings are not driving the top as much as they would be at higher tuning (and therefore higher tension). There is also less noticable 'bark' or 'honk' in the midrange frquencies when tuning down. Some may prefer this. I am not sure if I do, as I found it hard to adjust my fretting technique as per below, and play consistently.
Tuning down like this you have to be very careful of your string fretting pressure and to not BEND then notes when forming chords since you are working with lower tension, it is easier to make all your fretted notes go sharp (also by pressing down towards the fingerboard more easily), but the nice thing is that the significantly LOWER string tension lets you play with a very light touch on your fretting hand, which may be helpful if you have arthritis or similar problems in your hands.
Right now on the tenor Flea, I've got a set of Aquila Nylguts tuned to Bb tuning, and arriving in the mail today are a set of D'Addario J71 nylons (Jake's set) and a set of Jim Dunlop tenor strings, which I plan to try out as well. My tenor Fluke is very happy with the Aquila REDS re-entrant, in Bb tuning, and no REDS have popped since I strung it and tuned it this way like 2 months ago (now that I said that in public
I am afraid that Murphy will intervene), but I wanted a different tone and feel on the Flea, so...
:shaka: