PLT OF A TWIN TURBOPROP FLEW A ROUND ROBIN TRIP THROUGH AN ADIZ ZONE WITHOUT A XPONDER ON WITH ASSIGNED CODE AND ALT DUE TO SELECTING THE #2 XPONDER WHICH WAS IN STANDBY.

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I WAS THE PIC OF A FLT THAT RETURNED TO NOME; AK (OME); FROM GAMBELL; AK (GAM); IN AN ADIZ ON A ROUND ROBIN DVFR FLT PLAN WITH NO XPONDER. NOME FSS INFORMED ME WHEN I CALLED IN FOR AN ARPT ADVISORY. THE #1 XPONDER WAS ON THE PROPER CODE; AND IN ALT MODE; BUT THE SELECTOR SWITCH WAS SELECTED TO #2 XPONDER WHICH WAS IN STANDBY. AS I APCHED GAM ON THE OME-GAM LEG; A SNOWSHOWER MOVED OVER THE GAM ARPT REDUCING VISIBILITY. I CHANGED DME FROM #1 TO #2; WHICH WAS TUNED TO GAM FOR A HOLDING POS. AS I WAS SWITCHING DME'S; I COULD SEE THE VILLAGE AND ARPT APPEAR BEHIND THE SHOWER AND DID NOT BOTHER TO VERIFY THE DME. ON THE RETURN LEG TO OME; AFTER LEVELING OFF AT 9500 FT; THE ACFT GYRO PRESSURIZATION DROPPED TO ZERO PSI; FAILING ALL OF THE AIR DRIVEN GYROS. THIS WAS NOT A PROB FOR THE FLT BECAUSE OF GOOD VMC FOR THE FLT. I WAS PREOCCUPIED WITH TROUBLESHOOTING THE GYRO PRESSURE PROB AND DID NOT VERIFY THE XPONDER REPLY LIGHT. I MUST HAVE SELECTED #2 XPONDER INSTEAD OF #2 DME WHEN APCHING GAM. ADHERING TO THE BASIC RULE OF VERIFYING EVERYTHING YOU DO WITH A SWITCH OR CTL REALLY WOULD HAVE HELPED TO CAUSE WHAT I WANTED TO HAPPEN. BEING PREOCCUPIED WITH A PROB PREVENTED WHAT I WANTED TO HAPPEN.