Smart gate and e-gate are two separate things.

Smart gate is the easy entry system, where you scan a machine readable passport and use facial recognition and eye scan to enter. After you pass through the first time (go through the non reigstered queue), you'll be registered manually by an immigration officer, then the next time you enter Dubai, you can use the registered queue. There is no cost to this. This is definitely at T3 arrivals - it should be at T1 arrivals when it's all complete, but not sure if it's done yet. I'm sure someone who has been through there recently will let us know!

The smart gates, however, also accept UAE e-gate cards and UAE e-gate enabled identity cards. They usually use a separate validation system, fingerprints and you need to be pre-registered at a DNRD office, which you can find around Dubai or at the airport. If you have a physical Skywards card, you can use that as an e-gate card free of charge, or you can pay I think 325 AED for one. Suffice to say, it is probably best to use your machine readable passport.

I have to admit I have not left DXB except through an e-gate or desk, so I am not sure if T1 or T3 has smart gates on exit - I have only seen e-gates, which require the RFID e-gate card and fingerprint registration to work (there's also a 6-8 hour cool-off period too, so sometimes you have to see an officer anyway). I suspect smart gates are only for entering Dubai, not leaving.