You can use a trigger. Place it on the map right next to the fuel station and hit F2, then drag the thin line to the gas station itself. the trigger will say "static object" and click "not present." The gas station must be within the trigger's radius; hence when it's destroyed it won't be there anymore and the trigger will fire.
Then in the "on activation" field, type
"objective_1" objstatus "DONE"
where "objective_1" is whatever you named the objective in the briefing file.