Can I use pumpkin instead of ash gourd?

No. Pumpkin is very different from ash gourd. 

Ash gourd is also called 'winter melon', 'winter gourd' in English. It is called 'Saphed Petha' in Hindi. It can be found at vegetable vendors in India. It is a great vegetable to juice as it's juice is highly detoxifying. If you don't have access to ash gourd, use any other vegetable like bottle gourd, cucumber, celery, etc. 

Ash gourd looks like this -