This is the launchd.plist file that keeps my Dropbox and Google Drive folders synchronized.

gdrive-dropbox-sync.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Label</key><string>local.gdrive-dropbox-sync</string>
    <key>ProgramArguments</key>
    <array>
        <string>/bin/sh</string><string>-c</string>
        <string>/usr/local/bin/unison \
                  -times \
                  -prefer=newer \
                  -owner \
                  -group \
                  -ignorelocks \
                  -terse \
                  -batch \
                  -ignore='Path .dropbox.cache' \
                  -ignore='Path .DS_Store' \
                  -ignore='Path Icon?' \
                  ~/Google\ Drive/ \
                  ~/Dropbox/</string>
    </array>
    <key>RunAtLoad</key><true/>
    <key>WatchPaths</key>
    <array>
        <string>~/Google Drive</string>
        <string>~/Dropbox</string>
    </array>
<!--<key>StartInterval</key><integer>60</integer>-->
    <key>ExitTimeOut</key><integer>0</integer>
    <key>ThrottleInterval</key><integer>1</integer>
</dict>
</plist>

You need the unison command-line tool installed first - get it via home-brew (brew install unison). Then copy this file to ~/Library/LaunchAgents/ (it should be owned by your user, not root, and permissions should be 644) and run launchctl load -w ~/Library/LaunchAgents/gdrive-dropbox-sync.plist (without sudo) to start it up. It will monitor ~/Google Drive and ~/Dropbox for changes and propagate them. Logs will appear in ~/unison.log

As of OS X Yosemite (v10.10), the EnableGlobbing key is no longer supported. This means that the ~/Google Drive/ and ~/Dropbox arguments to unison will not be expanded to the user home directory (though the arguments to WatchPaths still seem to handle ~ correctly...)

Consequently, I have updated the plist to run unison via /bin/sh so as to allow home-directory expansion.

It should now once again function under the latest OSX version.

Thank you! Saved me alot of confusion getting sync to work with unison. Exactly what I was looking for.