1. As others have said, using penetrating oil - GT85 can dissolve rust well enough that you may get the bolt turning. You may want to turn the bike upside to shoot some GT85 into he underneath of the steerer. Leave the oil to soak in to the corroded surface of the metal for a few hours, then apply a bit more. Clean up the drips with a rag. Place another rag on the floor to catch any drips.
2. Find a properly fitting spanner, not an adjustable one, with a nice long handle. You might be able to use a six sided socket head with suitable handle, which is less likely to deform the bolt head. It can help to have the wheels (with tyres) installed when working on the bolt. You grip the front wheel between your knees as you turn the spanner. Anti-clockwise to unscrew the bolt (when viewed from above).
3. As explained above, once you get half an inch of elevation on that bolt, apply the hammer to it, directly downwards. A few strikes with increasing force. This may be enough to release the corroded expander cone inside. If it's worked, the bolt will suddenly come loose. Keep fingers away from the bolt head when using the hammer.
4. The stem body may still be stuck in place. If so. apply a little more penetrating oil (which might get to the stem body now that the expander cone is dislodged). Mount as long a handlebar as you have in the stem. Hold the wheel between your knees and turn the handlebar abruptly, left and right repeatedly. This may loosen the stem body a few degrees. Carry on turning the handlebar while pulling it upwards, to wiggle the stem up and out. This final part requires strength, control and patience. Take a break and have another go later, if you get tired. Keep your face away from the stem while pulling it up so the stem doesn't hit your face when it comes free.
5. If the expander cone is still lodged in the steerer, knock it up and out from beneath using a suitable drift / steel tube / centre punch or similar tool. You'll insert the drift into the bottom of the steerer until you locate the expander cone. Then tap the end of the drift with your hammer. The expander should rise up the inside of the steerer little by little.
6. The inside of the steerer will be rusty. You'll want to clean it before inserting another stem. This may be possible with a tough cloth such as a piece of denim, with GT85 on it.