This one is covered in the latest iteration of the rulebook under section 7.1.2 inventing gadgets the paragraph thats covers it reads as follows.
If the resulting skill check equals or exceeds the Gadget’s difficulty, the invention was a success
and the Gadget enters play. Attach it to the Mad Scientist unless the Gadget says otherwise. If
the skill test fails, the Gadget is placed in your discard pile.