I wanted to know is it permissable for a man to go to a gym or go to a boxing class where it is mixed.
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
It is not permissible to go to any place (gym, boxing classes etc) where there is free intermingling of men and women. 
And Allah Ta’āla Knows Best
Student Darul Iftaa
Checked and Approved by,
Mufti Ebrahim Desai.