Gslapt sees the ffmpeg 0.6 version as a downgrade, if you attempt to remove the svn_13024 version it will remove most multimedia apps. I could downgrade but have no idea if 0.6 will have libmp3lame built in.
Appearing as a downgrade is a rather dubious "feature" of the way Gslapt works. What you need to do is start Gslapt and go to Edit, Preferences, Sources. For each source that has a dark box click on each one and hit Edit. Priority should be Default. If something else shows in Priority, change it to Default. OK your way out and click on the Update icon. Now you won't get a "downgrade" when it's actually a newer version.
As for uninstalling, I'll say categorically that you should *never* do it with Gslapt. Always uninstall just the packages you want to remove by opening a terminal, doing su to root and then type
(putting the actual package name instead of nameofpackage).
Gslapt is often much too aggressive in removing packages and will take out other packages that might use the package you are removing but don't actually need it. removepkg is less aggressive.