Unless you're living in a very religious, closed community, I would personally avoid Faith and Grace as the pair. And I personally think themes are for nurseries, not humans, but Grace and Gretchen isn't a bad combo. I would stop there, though and not keep using G's for future kids (or this one if you and DH are G folk too).
I don't mind subtle themes but Grace and Faith is a bit of an eyeroll. If it was something like Grace and Felicity, for example, I wouldn't find it so cheesy. Otherwise, Gretchen.
Grace and Faith are great alone but cheesy together. Letter themes don't bother me until the 4th kid... then it gets weird IMO. I like Gretchen a lot. Also Gwen and Greta.
Re: Gretchen or Faith?
TTC since September 2012
Otherwise, Gretchen.
Baby #2 Due 3/7/20
My furchildren, Dorothy and Johnny