Predecessor Builds > Gadget > Gadget poking mines

For this gadget build you want to maximize the amount of damage you do with your Q, sticky mines. First you maximize Q for maximum poking, then maximize rmb for maximum wave clear, then E.

We start with Timewarp since it provides magical power, mana regeneration AND a lot of cooldown reduction. Three things that are very helpful in maximizing your poke with Q.
Following on that, we take Wraith Leggings. This item helps with securing kills, as damage on enemies below 40% is amplified. Furthermore, it provides some safety as the movement speed that you have in fights will make it harder to target you, while it also makes it easier to secure kills. The magic pen is also helpful here.
Oblivion Crown is extremely effective at maximizing your magical power. The passive increasing your magical power with 15% becomes more effective the more power you have, and this, together with its costs, means we will take this a bit later in our build.
The last two items are Caustica and Astral Catalyst, but these items can be exchanged for other items as well. Caustica provides some mana regen that keeps Gadget around longer and gives great magic pen. Astral Catalyst doesn't add great power, but it does add to your cooldowns, including a bonus on the cooldown of your strong ultimate. It also adds a nice amount of mana.
There are a few items that can be considered in this build given certain situations. Tainted Scepter is important to consider against a team with a lot of lifesteal. Besides blight it provides further utility by enhancing poke, adding health and adding cooldown reduction. Megacosm is great at enhancing damage dealt, especially on tanky heroes. This item can be added when dealing with a fairly tanky team. Golem's gift provides a great defence against burst attacks of physical damage, giving you time to escape alive. This is fairly useful against many fighters, which you can trap in your E while escaping after you survived the burst. Lastly, spellbreaker is a great defence against magical burst opponents. Don't use this item against opponents like Fey or Argus, however, as these heroes have an easy time removing the spell shield which makes this item good.
As a crest item, I'd recommend Soulbearer, as this item combined with your ult can give you a good amount of health from lifesteal, and this together with the shield makes you even more slippery than you already were.