RPI-arduino communication without UART
What is the data rate at which Pi (needs to) speaks to Arduino?  

what is the data size?  

what is the data interval?   

Can Pi be interrupted by GPIO and act like a Slave?   

Or Signal Arduino to switch to I2C slave mode.. 

Solution 1:

you can also use 2:1 Mux(analog switches) to multiplex UART lines between two devices for example..
 when Pi is using UART for GPS module, the two UART lines of Pi gets connected to GPS module. When there is no data to be received The same UART lines can be connected to Arduino. One extra IC and one GPIO from Pi is needed for this.  

Solution 2:
Arduino can control both Pi and the sensor. Pi can be configured to act like slave. Pi can interrupt Arduino via GPIO whenever Pi has something to communicate.