David Johnson-Davies designed a GPS module that decodes the NMEA sentences on ATtiny841, and this also runs the I2C slave routine. He writes:
This is a compact GPS module with an I2C interface, making it easy to read the GPS parameters from another project without needing to worry about parsing the NMEA sentences. It’s a PCB-mounted version of my earlier project I2C GPS Module:
As an example of using the module I also describe a GPS mapping application, that plots a map of your route on an OLED display.
I based the PCB on the GlobalTop Technology FGPMMOPA6C GPS module, also known as the GTPA010 [1], and these are available from Aliexpress [2]. It should also work with more recent versions of that module, such as the PA6H available from Adafruit [3]. The GPS module is rated at 3.3V so I included a 3.3V regulator on the board so you can power the board from 3.3V or 5V.
I2C GPS Module with NMEA decoding – [Link]