The pilot of a light turbine twin reports attempting to fly the ILS to Runway 10L at PDX with the course display set to GPS instead of VOR/LOC. Reporter descends to VMC below 700 feet MSL just west of the Interstate 5 bridge towers resulting in a low altitude call from ATC.

Narrative

I recently checked out and have less than 40 hours in the aircraft. I departed under IMC as a single pilot for a short flight in high workload IMC to PDX. The weather was low overcast and the temperature was near freezing. I departed with instructions to level at 4000 FT MSL. Shortly after departure I was switched to PDX Approach Control who gave me a vector to intercept the localizer for Runway 10L. I had all the correct frequencies tuned and identified; I had the correct weather and had briefed myself on the approach into PDX. The weather was about 800 FT MSL overcast and 2 miles visibility. I remained on the vector until I flew through the localizer and received a query from ATC as to my intentions. My HSI showed a full deflection yet the GPS screen showed me flying through it. I realized too late that I had my Garmin GNS 530 W set on GPS mode instead of VLOC mode and therefore I was not receiving correct navigation information for the ILS approach. Rather than fly a missed approach and risk being late to pickup my passengers; I attempted to follow the GPS course indicated on the screen rather than switch from GPS mode to VLOC mode and recapture. I descended below the minimum altitude at the marker for Runway 10L had got an altitude warning from ATC telling me to pull up immediately. I broke out around 700 FT MSL and was within a few hundred feet of the interstate 5 bridge towers. The bottom line is that was really stupid and I should have gone missed or asked for vectors immediately as soon as I realized I flown through the localizer for Runway 10L. I could have had ample time to reset the approach in the correct navigation mode. Since that incident I have amended my checklist to include a 'GPS MODE' verification check on the before takeoff check as well as the before landing checklist. I believe this will help fix any future problems as long as I'm diligent with my checklist usage.