There are lots of websites about gadgets, but...Porthole combines with Pot w/ Lid to make Port-o-Potty (serious), which is one of the Gadgeteer's most useful items (early on at least, it does Noxious Fumes)
Damaged Black Box is only there to give a clue that you can examine Black Boxes (like the one from the Helazoid ship)
Magnifying glass combines with Ptorch to make Searchlight (Detect Secrets)
Lightning Rod, can be used to do some minor damage, doesn't need to be combined with anything
Toy Doll combines with Hinged Box to make Jack-in-the-Box, which does Terror. Note that some websites mention you can combine this further still with the Demon Doll, but this last gadget was left out of the game.
Gears and Parts... huh???
Note that you need a varying Engineering skill in order to combine Gadget items.