Depending on what type of floor boards you have fitted depends on weather the gap should be there.Alot of floorboards are of a tongue and groove type so the "gap" should be there.As for restoring them ,hire a floor sander from a local DIY hire shop but remember to make sure that before you sand to go round and tap any nails down so they dont rip the sanding belt.I've heard of several methods of sanding but i've found the best method to sand is at a 45 degree angle accross the boards.To save on cost I used an angle grinder with a sanding pad fitted to go round the edges rather than hiring an edge sander(I own the angle grinder) After sanding you can either stain or varnish your floor depending on personal preference,clear varnish looks great ,ive done all my floors like this !