Sorry, to be more specific, you have to have been through the point in which Chloe and Chet take you to the hospital and has a doctor exam you, then take you back home and Chloe pegs you. You also need to have brought Gabby the Trinket from the park. Once you're at (or past) that point, you will have the option 'Mention Gabby to Chloe' when talking to her in her room.
I hope that makes sense