The gallery is very thin. It cannot take much leverage. If stuck, use Penetrating oil, and let it soak in, more penetrating oil, then freezing. Then try it. The gallery can be stuck from two reasons: old oil residue, and rust/corrosion. Penetrating oil can infiltrate the old oil residue, given time, and can break the rust/corrosion barrier, given time, but it will still be difficult to turn in the threads. Remember that metal expands and contracts with heat and cold, respectively. By placing the font/gallery assembly in a freezer at 0F, the metal will shrink and further weaken the bond binding the threads. If that doesn't free it up, then the application of heat might well do it. But brute force applied without giving any type of penetrating oil time to work will almost always result in a bent gallery. Patience will persist here.
If the gallery and fount are not rusted, merely glued together from old solidified fuel residue, then boiling in detergent and hot water is the easiest way to get parts "unglued" so they can be removed without damage. There is a procedure here on Miles Stair's website.
I have also had luck gently prying the gallery with a small screwdriver. This is done EVER so slightly so you don't damage the font. As I pry up.. I spray penetrating oil .. and work my self all the way around.. many times this breaks the rust or solids free. Allowing the Gallery to unscrew.