For the first part, yes, you can easily recreate the gateway shield in any footprint you would like. The schematics and Eagle files are available and all the parts (except the module) can be purchased on digikey. So you should pretty easily be able to make your own gateway.
As for the Electron, you can certainly hook up other sensors, you just need to be aware of the pins/peripherals we already use. So we do use SPI to talk from the Electron to the bluz module, and we use up 7 pins (including the standard SPI and UART pins). You can view the schematic for the exact details, but as long as you can live with these limitations, you can hook up the Electron to any other sensor.
As for processing, you wouldn’t want to hook up any sensor that would block other code for a long period of time. The gateway needs to forward all bluz traffic to the cloud, and if that gets blocked, it could interrupt the bluz communication. You can get around that by using local communication (which is recommended with an Electron anyway) as it doesn’t have to process the data for the cloud.