But after pg MSc honours geology can I apply ISRO 

Hello Ruchi,

Yes, you can very much get admission in DU for BSc(H)Geology. You got a good percentage and you'll get it easily. Good NEWS!! There is no disadvantage for gap year students. This is what Delhi University says about the gap year students:

"No, there is no disadvantage for gap year students and they would be treated at par with regular students. They should meet the requisite criteria for admission and must possess the necessary documents/certificates."

Best Wishes!