what kind of API gateway are you using, REST or HTTP?
Here is an explanation between their difference:
https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-vs-rest.html
If you are using the newer style HTTP one, you have to explicitly configure the lambda integration, you are assigning to a route, to use the version 1.0 payload format.
That’s a common problem, so I’m mentioning it without looking into the details you shared.
What was the response you got?
I’m offered to download something, when I clicked on your API gateway link, but not sure what, because I just tried it on an iOS device.