In a radical departure from my usual affinity for minor characters, I have to say Riley is my favorite.
I'm the youngest of three boys and both my parents were teachers. My forth grade year my mom went back to work, and coincidentally, the school district lines were redrawn. Our home was the literally the last house in the new district and I started going to a different school. It took a couple months for me to adjust. I thought about going to live in a tree house me and my friends built because it was near the school I used to attend. Fortunately, the odd collection of food stuff my friends took from their homes made me realize it wouldn't work and I went back home before that day's dinner.
I had forgotten about that, but seeing Riley's emotional journey brought it all back.
Gloom isn't mentioned by anyone and does not appear in the movie other than being a shadow that is seem moving across various locations.
It is mentioned in some of the books that are Inside Out themed. The article on our wiki is largely speculative and has been left in tact because no-one has come forth with better information.