There is a project that uses a Raspberry pi to transmit from is GPIO
To use a home built transmitter you need amateur radio license, no matter what.
Instead: To use a home built transmitter you need amateur radio license, except in certain specific low power situations. To comply with those situations would be difficult without specialised test gear.
https://www.fcc.gov/media/radio/low-power-radio-general-info...
For any of the fun stuff, you need an amateur radio license :-)
This is absolutely key here - even in unlicensed bands one cannot just start using the spectrum without careful consideration of regulations for that spectrum.
When I was working in the 902-928 Mhz (FCC and similar) and 860s (for ETSI) for RFID we had to do a lot of work to ensure our transmit power, power ramp up, frequency hopping and listen before talk were all implemented properly and demonstrable for certification. It was not trivial.