Yeah custom gpts are only for paid users. However u can create a new project under "Projects", name it, then when u create it, you can see on the top right the three dots button, click it, open project settings, and there u can place your system prompt under instructions. Every chat you start in that project would send those instructions as a system prompt to the model you are chatting with. so essentially "Study Mode" could be recreated with this approach, or at least it should.