This Seeking Tech guide will explain how you can get the Sonic Jam DLC skin for free in Sonic X Shadow Generations.
In order to get this costume, you need to sign up for Sega’s email newsletter by November 1st. During the process, make sure you select the platform where you will be playing Sonic X Shadow Generations.
After signing up, you need to verify your email address.
Sega should then email a code with instructions for redeeming the Sonic Jam DLC skin.
You can signup for the DLC with the following link: https://sonicxshadowgenerations.com/emailsignup/index.html?lang=en