MCA

https://www.shiksha.com/search/?q=MCA&bc[]=103&sa=^106^0^103^0^0^0^0^0^0^0^0^0^0^bc:103&rf=searchwidget&st[]=106.

You need to have 50% to be eligible to get admission in this course and college. Candidates who are from non-Computer Science, should have studied any of the following subjects at the Pre- University or Degree level Mathematics, Computer Science, Business Mathematics or Electronics.

Yes, you can apply for GATE. As per eligibility criteria, a candidate must be passed out or in final semester of the M.Sc/MA/MCA course, there is no minimum and maximum age limit for GATE. There are 24 subjects papers in GATE exam and one has required to choose only one from them. As you have completed yours MCA. So, you can opt for CS (Computer Science and information technology) subject code. GOOD LUCK!

In about salary- It depends on the company and your achievement qualification and knowledge.

No, for MCA candidate you must have their bachelor's in IT or CS.

Hi,
You can consider these colleges:
University of Delhi
National Institute of Technology Tiruchirappalli
Banaras Hindu University, Varanasi
Guru Nanak Dev University, Amritsar (Punjab)
Andhra University Colleges of Engineering
Cochin University of Science and Technology
Kurukshetra University.

As the fees is not disclosed by the college, request you to contact the college officials for the same.
Link: http://eng.bangaloreuniversity.ac.in/

KIIT School of computer applications offers two years of MCA Lateral entry program. The eligibility criteria for this program is graduation in relevant discipline. Further your rank is not enough for this program, may please improve this.

There are various MCA government colleges in Gujarat. You can go through this link to find out in more detail:
https://www.shiksha.com/it-software/colleges/mca-colleges-gujarat.