This is very interesting, and I am surprised it didn't happen a long time ago. The Lambda function integration opens up lots of new ideas when building API backends ready to be consumed by clients apps like, for example, a client-side Javascript application.
On the other side it seems like other extra functionality is limited and very AWS-oriented. If you are looking for an open source gateway that can be expanded with extra functionality, and potentially sit on top of the AWS-> Lambda integration, take a look at https://github.com/Mashape/kong (I am a core maintainer too, so feel free to ask me any question). Kong is accepting plugins from the community (http://getkong.org/plugins), which can be used to replace or extend functionality beyond any other gateway including AWS Gateway.
On the other side it seems like other extra functionality is limited and very AWS-oriented. If you are looking for an open source gateway that can be expanded with extra functionality, and potentially sit on top of the AWS-> Lambda integration, take a look at https://github.com/Mashape/kong (I am a core maintainer too, so feel free to ask me any question). Kong is accepting plugins from the community (http://getkong.org/plugins), which can be used to replace or extend functionality beyond any other gateway including AWS Gateway.