What gaps are you talking about? What is your main concern here?
If you are replacing the tub the you have to rip wall finishes off down to studs. Remove toilet and vanity so there is room to work and maneuver tub.
Get a tub that is the same size as the old. When you set the new tub in use some structolite(cement plaster material) to create a solid molded base under the tub. The new tub should have a flange lip that the cement board goes up to and then tiled over.
If you have no bathroom remodeling experience do not practice on this bathroom. Don't know if I agree with wooden mouldings around the window in shower area. The are materials to waterproof around the window before you tile it.
Optional: Build wall out 6 inches(the wall with the medicine cabinet) only in the shower area and install standard 5 foot tub. You'll need a plumber to hook up the drain, water pipes, and fixtures. If it were me I gut everything to the studs and floor joists and change all the supply and drain lines.