That's strange that it wouldn't work! What device do you have?
Did you try to empty the cache for Google Download Manager, Google +, Gmail and Gallery? While you are at it, do the Google Play Store as well. All of these can be found under Settings, Apps (or Application Manager) and then All.
Deactivate and activate (disable and enable / turn off and turn on) the Download manager, Gallery, Gmail and Google+ after this.
You can also remove your Google Account if you go under Settings, then under Accounts and choose Google. Remove the account. After this is done, you can add it again.
The last option would be to do a factory reset, but in order to not lose your data, it's super important to do a back up of your pictures, contacts etc. Try the first options first before doing this and ask again if you have any more questions!