ACCORDING TO THE PLT, THE WX FOR HIS ROUTE OF FLT WAS FORCAST TO REMAIN VFR. EN ROUTE, CEILINGS AT 4000 TO 5000 FT MSL WERE ENCOUNTERED IN THE VICINITY OF HOBART, OK. AS THE FLT CONTINUED THE WX DETERIORATED. IN ORDER TO MAINTAIN VFR CONDITIONS, THE NON-INSTRUMENT RATED PLT DESCENDED TO 3200 FT MSL, THEN TO 3000 FT MSL, & CHANGED HIS DESTINATION FROM WOODWARD, OK, TO GAGE, OK. AS THE CEILING LOWERED, THE PLT SLOWED THE ACFT TO 100 TO 120 MPH & CRUISED NEAR AN INDICATED 2500 FT (AT NIGHT). HE THOUGHT HE WAS CRUISING BELOW THE CLOUDS AT 200 TO 300 FT AGL. SUBSEQUENTLY, THE ACFT COLLIDED WITH THE TERRAIN IN A WINGS LEVEL, FLAT ATTITUDE AT A REPORTED ELEVATION OF 2473 FT. THE WX REPORTING FACILITIES AT GAGE & HOBART HAD CLOSED AT 2100 & 1800 CDT, RESPECTIVELY. FT SILL, OK, REPORTED IFR CEILINGS OF 700 FT OVERCAST WITH 10 MI VISIBILITY. THERE WAS NO FSS RECORD OF THE PLT UPDATING HIS WX BRIEFING WHILE EN ROUTE.