Your query is quite complicated as it is not clear. On one hand, you are saying that "you have been working in an organization since 15/6/2007 and you think 1 or 2 years later you will leave PF & ESIC," this is not clear; what does it mean?
The thing that is clear and can be replied to is that "you have been working in an organization since 15/6/2007 but they didn't give you any appointment letter. You have a salary slip from the first month, and they give you a cheque, then they are transferring your salary to your account till the present," if I am getting it right. Please correct me if there is anything wrong.
In the above situation, as your service is now over 6 years, you are eligible for gratuity. The fact that your employer has not given you an appointment letter will not matter there, as you can say that they didn't give you an appointment letter.
Firstly, as they paid your first salary through a cheque and thereafter through bank transfer, and you have a salary slip, this is sufficient to prove that you were their employee.
Secondly, the PF & ESI point you have mentioned is not clear. But if you are covered under ESI & EPF and you know your ESI IP No. and PF A/c No., then through this, you can also prove that you are their employee.
Although this is just your assumption that they might not pay you gratuity, I don't think so, as they also know all the above facts. But if your employer still denies to pay you gratuity, then you can firstly submit your Gratuity Claim through Registered Post with a CC to the Labour Department (in whose ambit your location falls) and if they don't settle your gratuity, then you can lodge a complaint under The Payment of Gratuity Act, to the Labour Department.
Hope this information will be helpful to you. Be Happy.
Regards