Sorry this is a bit badly worded, let me explain the difference between Biller and Gateway, and we will use these questions as well to make changes to those names in the future:
Biller (we should name this "Direct" moving forward):
If you are using Cashtie API for your own app, you want to sign up as a biller. An example would be you are adding Cashtie barcodes to your bill pay application and have consumers pay their bills at a cash register.
If you are building a solution that others can use together with the Cashtie API you are a Gateway. An example is you are building a plugin for Magento or Shopify to allow ecommerce cash payments via Cashtie and now the ecommerce store owners start using your plugin with Cashtie API.