Dipper and Mabel's Guide To Mystery and Non-Stop Fun: Secret Codes (Sorry this took so long, BTW.) Here are the codes I have found in Dipper and Mabel’s Guide To Mystery and Non-Stop Fun.
Gravity Falls: Break the Code!<<Not only do I know this code, but I have memorized it and can write almost fluently.
Disney Bill Cipher presents.
Dipper from gravity falls Kyle could cosplay him!