They might be using the hotel gauging currency conversion rate to convert the local currency to USD, then charged that amount to your card.

Several years ago when I tried to use my GC at Skycity in Hong Kong and Sheraton in Macau - both properties told me the only way they processed GC would be to do currency conversion, at the hotel's own exchange rates which of course were very unfavorable. So I paid with CC instead. I have not had a single success in using GC successfully outside US. Even inside US, a local Sheraton insisted they could not use the physical card bought from a local Ren, but they could use eGC... It should be the same process from what I understand. But the FD Manager insisted the system told him GC not recognized... and then he kept pointing that it was for Ren, not for Sheraton... <roll eyes>