I forgot to wash inner belly button for Ghusl should I assume water went in there when I showered
The majority of scholars are of the opinion that water reaching all parts of the body is a condition for the validity of ghusl , this includes making sure that water reaches the navel .
Therefore, if you fear that you forgot some part(s) of the body, or can't be reasonably sure that water reached everywhere, you should redo the part of ghusl that you fear that you may have missed . In your specific case, it sounds like you assumed it reached your inner belly button, but you didn't actively wash there, which appears to me to adhere to the second of aforementioned scenarios for when one should redo the missed part.
And Allaah Azza Wa Jalla knows best.