You have to meet the following eligibility criteria to wite the GATE Exam-

1. NATIONALITY

• Candidate should be an Indian National to appear for GATE 2019
  
• Candidates belonging to Bangladesh, Sri Lanka, Singapore, Nepal, Ethiopia and United Arab Emirates (U.A.E) will also be eligible to appear for GATE 2019.
  

2. ACADEMIC QUALIFICATIONS

• The candidate should have completed his/her Bachelor's degree in Engineering/Technology or

Master’s degree (M.Sc) in any relevant science subject or must be in the final year of the programme.

• For GATE there is no minimum pass percentage in the qualifying degree
  

So, people who have done their graduation in BCOM and postgraduation in MBA can not apply or are not eligible to write GATE exam. Infact, a person from non science backgrund is not eligible for GATE exam.

I hope this was helpful!

• Candidate should have completed their Bachelor’s degree in Engineering/Technology or Master’s degree (M.Sc) in any relevant science subject or must be in the final year of the programme.
•  for GATE 2019, there is no minimum pass percentage in the qualifying degree.

Hii,

No you can't write GATE exam by pursuing Bcom course. Graduate Aptitude Test of Engineering is only for the graduates who completed Bachelor of Technology or Bachelor of Engineering in any Specialization. The final year students can also write the exam .