It's gray to blend in with sheet metal ducting. there's really no reason it has to be gray though, with plenty of brands offering other colors or patterns. It's just tradition at this point- you expect duct tape to be gray, so they make it gray.

What makes duck or duct tap duct tape is that it's a pressure sensitive adhesive, with a fabric backing and some kind of plastic layer to "waterproof" it, as well as a fabric that's woven in such a way as to be tearable by hand (and not shears, like other fabric tapes of the time.)

this form was originally developed during WW2 as a means of sealing ammo cans for the war, and was the army's olive drab in color.

it switched to gray in color after WW2 when it became common to use it to wrap/seal air ducts.

As for why it sticks to everything better than "normal" tape, it depends on what you mean by "normal" tape. If you mean cellophane-packed tape (the clear plastic stuff, typically called scotch tape or packing tape), it doesn't actually. It just has more adhesive (which is impregnated into the fabric backing). the adhesive is actually (basically) the same pressure sensitive stuff. That is to say, the adhesive in the tape is activated by applying pressure to the tape; rather than a solvent that evaporates off (like glues, CA, Elmers, etc).

If you mean something like masking or painter's tape, or gaffer's tape, it's because the tape is designed with a different adhessive and these are meant to be temporary.

also, just for the record, magnets would only stick to ferromagnetic materials (stuff that contains iron, basically.) but there's no adhesive involved. If you made a simple electromagnet by wrapping wire around an iron/steel nail, and touching the wire to a battery's terminals, when you turn it off it would stop being attracted to things and just pop off.