Dear Ramu,
If some employee plays games on PC then that is a misconduct. In this misconduct where is the question of gender coming in? Had it been a male staff, would it be a less severe misconduct?
If employee has time to play games on her PC means she may not have sufficient work. Have you done workload analysis of her work?
Do you have sufficient evidence for the misconduct? Have you called that employee in your office and given verbal warning? It is not desirable to give written warning before giving verbal warning.
Lastly, I recommend you writing warning letter on your own and uploading it here. Some senior will correct it.
If you want reference of one latest draft then you may
click here.
Dinesh V Divekar