No you cannot use 3rd-party gateways, you must use Google's ip-app purchase system. They recently updated their terms of use.
Developers offering additional content, services or functionality within another category of app downloaded from Google Play must use Google Play’s in-app billing service as the method of payment, except:
- where payment is primarily for physical goods or services (e.g. buying movie tickets, or buying a publication where the price also includes a hard copy subscription); or
- where payment is for digital content or goods that may be consumed outside of the application itself (e.g. buying songs that can be played on other music players).
Source: https://play.google.com/about/developer-content-policy.html