Why is my outlet voltage abnormal?
If A and B are the wires coming from the breaker, they should be connected to the LINE terminals of the GFCI.  C and D; that go off to serve other loads, should be connected to the LOAD terminals. If when connected properly, the GFCI will not reset. It means that the GFCI device is dead, and needs to be replaced.

Since C &amp; D are "floating", the 5.5 volts could be phantom voltage induced by other electrical lines in close proximity to these lines, or any other magnetic field. A low impedance voltmeter could be used to rule out phantom (ghost) voltage.  However, since lines C &amp; D have no reference to the electrical system (floating), don't expect a meaningful reading.