Actually you have a few options, if you make one of your Pis run Ubuntu there is a Gridcoin PPA that should be very easy https://launchpad.net/~gridcoin/+archive/ubuntu/gridcoin-stable . I didnt use it myself.
Another option is whats going on over at https://github.com/Steffov/GRC-Scripts . I didnt use it myself.
The compiling guide I wrote is pretty long, see about the others first and if they are no good for you perhaps I'll publish it on my own GitHub