Just moved in to an apartment with a gas stove which was not in use for three months. The previous owner said that the stove worked just fine, however when I tried to use the stove, all four knobs were stuck. Using WD40 i was able to release three knobs, but the fourth remains stuck. The gas company refuses to connect me unless all four knobs are working properly due to safety regulations, so I really need to release that stuck knob.
Try pulling the plastic knob off. There should be a metal peg sticking out (the plastic knob fits over it) then try squirting some WD-40 on it and gently grab the metal peg with some pliers and slowly turn it. Don't force it too much as it may break. When it loosens up, replace the peg. I hope this works for you. Good luck.