If you are certain you uploaded them to Google they should be there. However, there is a possibility that the person who stole your phone accessed and deleted them. There is two ways, that I can think of, that they could have done it.
First, if your phone was set to be always logged into your Google Account, they could have just entered it and deleted what was there.
Second, if your account was set to keep the phone and account synced, when/if they deleted the photos on the phone, it may have deleted the ones in the account. It would have probably prompted them to ask if they were sure they wanted to delete them.
Also, before giving up, you could check to see if they have a recovery option. I don't use Google so I can't see, but...