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.
I've tried WD40 and also Awesome (a strong cleaner). Nothing worked except soap chips. Run the round tube that fits over the gas tube and get some soap chips inside the hole that fits over the gas tube. It works.