Thanks for responding, everyone. Firstly, I do have WA EP installed and still cannot lock gates. And I have tried all gates in my game, including the Celeb gate with the call box attached. However, I haven't tried setting the front gate as the front door, so maybe that is the problem. I wasn't aware that setting gates as such was even possible--I've never seen that option before whenever I clicked on them.
The odd thing though, is that my Daughter can lock gates in her version and all she has is Pets EP, that's it. And she thought it was a base game feature, as it has always been available to her in her game. She got her game through Steam, while I bought mine as a bundled starter pack at Walmart. It came as a base game along with Late Night EP and HELS pack discs. My base game and all EP's have the latest patches.
I'll start a new game and set up a gated fenced-in yard and see if I'm able to set and lock the front gate.
I'll report back here with either a success story or a fail... :P