Bob,
I don't remember seeing the gate either. However, the day I was looking for the palace, the street was fairly crowded with teenagers. Perhaps the schools had just let out when I got there. Plus, I was very focused on finding #108 since that was what the guidebook listed as the address. But as you mentioned, the building at #108 looked like a palace, so I assumed it was the correct one. Plus it was undergoing renovation. It wasn't until I returned home that I found out Xenia's palace was located at #106.
If I ever have the opportunity to go back to Petersburg, I'll have to look into this again. Obviously, I missed out.
I will look through my photos to see if I caught the gates on film.
Roy (RGT9W)