Part of the WGT news for the July 7 release:
"In addition to a variety of ways to earn free WGT Credits, there are also new payment methods on WGT. Support for Google Checkout makes it easy to use your saved Google Checkout payment methods. And you now use Gift Cards on WGT, including Gift Cards from Subway, CVS and Circle K with more stores coming soon. Just enter your Gift Card number when you buy WGT Credits and you get 100% conversion (ie. $1 Subway equals $1 WGT)."
However, it appears that accepting purchases from CVS gift cards is "Coming Soon" rather than currently available.
Select credit amount, change payment method to gift card, no issues. However, when you get to the gift card info entry screen, there is no way to switch from Subway to some other gift card - CVS and Circle K are both listed as "Coming Soon".