Bhagwan Neminath attained Kevalgyan (omniscience) on Girnar mountain (Revatgiri / Ujjayantgiri) in Gujarat.

More specifically, Jain traditions mention:

• Place: Sahasavan (Sahasramar) forest on Mount Girnar, near Junagadh
• Kalyanaks there: Diksha, Kevalgyan and Moksha of Neminath Bhagwan all took place on Girnar.

You can read more about Girnar tirth here