What is the importance of an "RC filter array" in GPS module circuit design?
The RC filter is a low-pass filter and is meant to get rid of high frequency noise on the bus due to reflections.  The edges on an SPI bus can be fast enough that the resulting ringing can look like additional clock pulses, incorrect data, etc.


  Is it really necessary?


It depends.  The bus may communicate fine without it.  It's easier to place some sort of filtering in the comms lines before spinning the board rather than after.  You can always depopulate unneeded components, and/or replace with a 0-ohm until the footprint can be removed.  

If the question is whether it will damage anything if omitted, the answer is almost certainly 'no'.  You just run a higher chance of noisy SPI lines, and the fix will be messier if you don't have some sort of footprint for a filter already in place.

If in the end all you want is a more generic alternative rather to this specialized RC filter part, I've had some success with just putting a 22-Ohm resistor in series with each SPI line.  Place the resistor nearest where the signal originates.