A parent has ultimate responsibility for establishing and enforcing the rules and theories that apply to their children. Just as your parents did with you.
That said, a grandparent has many years of general experience and specific experience with you. You can evaluate their experience (and success or failure) by taking a look at yourself and remembering their parenting and what it meant and how it worked.
If you take the things they did that made you feel loved, worthwhile, valued and good and use them, or some version of them, in your own parenting then you can blend it in wtih your own style. (Be certain to exclude any of their parenting that made you feel bad, worthless or unloved.)
So, in a nutshell, you have the advantage of personally having seen your own parents (and those of your friends) in action. Take the best and merge it with your own approach. You do not have to reinvent the wheel.
Be flexible enough to reassess and make changes if warranted. In the end, however, you are the final authority on your children.