"If you are on an IFR flight plan and do not have an IFR certified GPS, can you use a VFR-only or handheld GPS for any portion of the enroute navigation?" - Al R.
“The short answer is no.
VFR and handheld GPS systems are not authorized for IFR navigation, instrument approaches, or as a primary instrument flight reference. During IFR operations they may be considered only an aid to situational awareness.
However, you may request or the controller may offer a RADAR vector to a fix. You can then use the GPS as an aid to maintain situational awareness during the course of the vector. Another approved navigation method must be used as well.”