Yes. Having this exact same problem. I tried in another mail client and apparently it's some kind of GMail security warning; it sees the new file format as a threat. I also tried attaching the file through GMail on the web, and it still failed.
I've posted this in several different places, including on Apple's discussion forums, and reported the issue to them (Although as of yesterday their feedback form had not been updated to include Pages 5.0).
I'm surprised I haven't seen more reported problems with this; emailing documents is a huge part of my workflow and this completely derails it.
Sure, it may be partially a Google problem - but obviously Apple has done something strange to the overall file format.
If anyone discovers a fix for this (other than converting the file to a different format)... please post it here!