Yes, a gap year after 12th does not disqualify you from SSC GD constable recruitment.
The eligibility conditions are only about age limits, educational qualification (10th pass), and physical standards — there is no continuous-study requirement.
You do not need to explain the gap in the application form.
Keep your 10th certificate and date-of-birth proof ready for document verification.
Apply only if you meet the age criteria on the cutoff date given in the official notification.