I would have to ask what notes of the open G tuning you are after. And 3, 4 or 5 string etc.
Gdg for electric, I use .042, .030, ..018. The .018 is unwould and sounds good for slide work.
I pull them from a set of D'Adario strings, EXL116
The high g string is tight for slide while the low G is floppy and dirty!
There made for drop tuning
I used to think this was largely a personal decision depending on playing style. Partially that's true. But after several builds I've discovered it's also the type of sound you want from a CBG.
For example within the tuning of GDg (standard CBG tuning) there are these possibilities:
ADG strings (A turned down 1 note to G)
ADe strings (A turned down and e turned up 1 1/2 notes to g)
ADb strings (A turned down and b turned down 1 1/2 notes to g)
Gbe strings (b tuned up 1 1/2 to d and e turned up 1 1/2 to g)
These different string uses will provide different sounds. The ADG provides a very rich, mellow tone. Gbe provides a high treble-sounding tone, almost like banjo, resonator or steel guitar.
And that isn't even taking into account string diameter. Then if we consider the popular alternate tuning of DGd... we bring the low E string into play and wind up with combinations such as EAD (E turned down to D and A down to G)... which really reverberates but is awful for slide (unless you use flatwound strings).
There's just so many combinations to choose from, I find myself unable to choose a "favorite" any more. The more I think about possible string and diameter combinations... the more CBGs I want to build to try them out.
I do agree with Jim about the D'Addario strings. They are my favorite brand. Whether you use phosphor bronze for piezo or their electrics for mags... great strings. The 16s seem to be "just about right"... and their middle-price range fits the pocketbook well.