I am working on a project that includes a GPS signal. In my electric diagram, do I have to add it? I don't know which is the symbol for that.
Nobody can force you to add a specific symbol to your schematic. So, no, you don't have to add it. The purpose of a schematic is to be clear and descriptive, not to be advertisement material.
I don't even know whether GPS has a symbol. NAVSTAR/GPS does have something like a coat of arms, but it's not an "official logo". After all, GPS is a US government program and not a profit-oriented enterprise.