Is this true that to reach Krishna we should go through channel of demigods. Are Ganapati and mother Durga demigods? What is difference between worshipping demigods and worshipping Krishna?

To reach Krishna we have to worship Krishna. The Bhagavad Gita explains that if we worship the demigods we go to them and if we worship Krishna we go to Him.

Ganapati and Durga are demigods.

The demigods are normally worshipped for material benedictions which also ultimately come from Krishna. Krishna is however worshipped to attain pure devotional service unto Him.