Yes, you can.

You can do a MTech in CSE after doing BTech in Electronics, but your GATE Paper must be Computer Science and IT. There is no problem in this process. Many students change their branches in Masters . If you want to study CSE you should.

For PES university, here is the eligibility criteria.

For admission to M. Tech programmes, an applicant possessing qualifications mentioned below is eligible to apply:

• The applicant must have a bachelor degree in engineering from a recognized university with a minimum aggregate of 60%
• The applicant should have a valid PESSAT, PGCET or GATE score to apply.

Hope it helps. Thank you.