Do I need to sand, condition, prime etc and what paint and stain is best to finish my Armoire?
Some sanding will be required to prep the surface.
If there's a film finish on the wood (varnish or a type of lacquer) it should probably be removed to give the best results, using a chemical stripper ideally. You can remove finish just with sandpaper but I don't recommend it, and it's nearly impossible to do on and around mouldings anyway.
You don't need to 'condition'.
There are also some marks where it looks like a dog chewed on the bottom as well and not sure how to deal with these.
Those would be tricky to hide completely even if you were painting the piece a solid colour, where the general procedure for the first-timer would be to use a commercial wood filler of some kind and then smoothing off by sanding.
With the finish you're looking to do the damage is likely to remain quite visible if tackled in this way I'm afraid. Replacement of the damaged wood with fresh wood is possibly the only entirely satisfactory solution.
Can I paint with grey and then use a gel stain or????
Yes you can. Gel stain brushed on and then with the excess wiped off using clean rags might give an effect quite like what you're looking for. The paint would need to be a lighter grey I think, but off-white might work OK depending on the gel stain you use and how it interacts with the paint. The gel stain should be black I'd say.
If using a waterbased paint under the varnish (gel stain is a type of varnish despite its name) allow ample time for the paint to dry, at least a day would be good and longer if it's cold and/or damp where you are.