Changing the cart notice w/ gateway module

I’m using the Custom Gateway by Category module and it’s working fine, it stops users checking out under the circumstances I input. However, I want to change this message:

“Sorry, it seems that there are no available payment methods for your state. Please contact us if you require assistance or wish to make alternate arrangements.”

Otherwise it makes no sense and users will not know why they can’t check out. Can anyone tell me where to change it? Thanks