If Peter had been looking for it, he might've recognized Gwen's voice and her mannerisms. He knew she was here, but this was the very last place he expected to find her. His Gwen hadn't been bitten, after all. So he had no reason to suspect anything.
As much as she was caught off guard about his orphan crack, Peter wasn't ready for her to know about his aunt. But it didn't give him pause right away. His initial reaction was to defend himself. "Hey, don't get me wrong! I love Aunt May! She's the best! But I do fit the dictionary definition of—" And that's when he finally paused.
Peter knew two people that knew both his true identity and his family and the fact that he was Spider-Man. This wasn't MJ, not with her anti-mask stance. He shook his head slightly, as in disbelief. "How do you know about my aunt?" he asked, though there was really only one way she could've known. He just needed to hear her say it.