My mil let my LO tried lemon. In general we let her do what she wants since she hardly get to see the LO plus she is a nurse. When I told my mum, she reprimanded me and said it would cause gastric. Would it?

It's okay for babies to try lemons! It won't harm them if it's in small quantities. The mom above is right, the pH value of lemon juice is no too bad and will not cause gastric problems if taken in small quantities. It's quite cute to see them react to the lemon too. Here's a cute video: https://www.youtube.com/watch?v=1YBheuHma8I