As long as your existing payment gateway is supported by Chargify already, then you can use it. Check our Payment Gateway List.
We support a number of payment gateways that work with banks/merchant accounts in the USA, Canada, UK, Australia, and New Zealand.
Some European merchants have been able to get a merchant account at Barclays Bank in the UK, and then use Payment Express in the UK.
If you'd like to learn all about merchant accounts & payment gateways, here is a Complete Description by one of our founders.