Can I do my own Survey If I use GPS?

GPS is not a legal method. You can not legally mark the boundries yourself. You should hunt them down though, they are usually set. Most surveyors use metal stakes.  You might need a metal detector to find it, they are often over grown or buried.

The only legal survey is done by a surveyor. Anything you come up is your opinion only and is not legal if you are building something that requires you to know where the property line is, fencing included. Any building also requires a set back.