If the employee has been granted leave by the company for more than 2 months, whether it is leave with full pay, half-pay leave, or any other type of leave for her marriage, then the employee is eligible to receive Gratuity. However, if the employee has been penalized for her absence resulting in a total tenure of less than 75% attendance over 4 years and/or less than 240 days attendance in the last year (i.e., the 5th year), then the employee is not entitled to Gratuity.
If your company policy does not permit granting a 2-month leave in a single stretch, you can advise her to return for 1 or 2 days after a gap of 1 month and then allow her to take another 1 month of leave. You can also use the day she returns for 1 or 2 days to organize a small office party to celebrate her marriage.
Remember, if you believe in something, everything is possible, but if you don't, then nothing is achievable.
Wishing you all the best.