Yes, you can join B.Com at Guru Nanak College Chennai if you have a science background. 

The college tends to prefer mostly students with an accounting background, as they will be already familiar with the concepts. But there are no criteria that only commerce students can opt for B.Com. Being a science student, you must have studied Mathematics, thus, you can easily catch up with the rest of the subjects too, if truly interested.

Doing B.Com from Guru Nanak College Chennai will definitely be worth a try if you have a keen interest in Mathematics.