Been playing with this today and mentioned this issue in the Telegram as well:

CMake Error at /opt/cmake/share/cmake-3.9/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
  Could NOT find GMP (missing: GMP_LIBRARIES GMP_INCLUDE_DIR)
Call Stack (most recent call first):
  /opt/cmake/share/cmake-3.9/Modules/FindPackageHandleStandardArgs.cmake:377 (_FPHSA_FAILURE_MESSAGE)
  libraries/fc/CMakeModules/FindGMP.cmake:50 (find_package_handle_standard_args)
  libraries/fc/CMakeLists.txt:20 (find_package)

I was told sudo apt-get install libgmp3-dev would do the trick, but it errored again, so now I'm attempting a build with --no-cache. I'm building on Google Compute Engine. Having fun so far. I hope I can get it going. :)

Edit: Ah, looks like a PR has been submitted and merged with a fix for this: https://github.com/EOSIO/eos/pull/105/files