Hello Senthil,
CCMT counselling has different groups for admissions to same disciplines of same colleges. These groups are based on the basis of eligibility discipline through which the candidate has done his bachelor degree and the GATE paper through which he or she has given.
So, regarding your question, there is no chance that a candidate belonging to g2 can opt for a seat in g1 except when the seats of the institute remain empty and the institute seeks to fill its seats.
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