It should be mentioned that those radio frequencies (60-66 Mhz for Channel 3) aren't for unlicensed use like WiFi. I'm sure no one will really care especially at low power, but getting a call from the FCC (or worse a visit from the FBI) would sorta suck.
Edit: Oh, nevermind. This is meant to be connected by a cable.
Shouldn't be an issue since the page mentions connecting the TV (ie via cable) so nothing is meant to be transmitted in the air, rather just as an input to the TV.
Same way as old NES consoles or C64 computers sent their signal to TV's before composite, SCART and later HDMI became the way to connect. This was often via a pass-through antenna connector that was placed between the regular antenna and TV-set (so you didn't have to disconnect it) and then just tuned one of the TV channels to the channel the console used (and iirc that channel was often channel 3 as mentioned in the github page).
Oh, I misunderstood the first line: "Hook an antenna up to GPIO3/RX". I didn't realize it was talking about the television antenna.
As an 80s kid, I'm quite familiar with the adapter for Channel 3. This is a cool project if you've got an old TV. There was a post about Radio Shack catalogs the other day and I still lust over a portable 2" color television from 1989, which would be useless today. I almost want to find one on eBay just to try this out.
People often freak out about these transmissions being against the law, but at least in the USA there are clear legal exemptions for hobbyist/experimental low power transmissions.
So don't worry about the sky falling, using this stuff is OK as long as you don't amplify it and it's in your interests to shield it if you can and just run it for brief periods of time - essentially, hobbyist/experimental usage.
Edit: Oh, nevermind. This is meant to be connected by a cable.