Predecessor Builds > Gadget > Quintessential Midlane Gadget Build
Key things to remember when building on Gadget:
The three core items of: Azure, Megacosm, Spellbreaker, give high ability haste, survivability and dmg verses the majority of the roster. You can build these 3 items vs any team comp and do well. It provides you anti-tank capabilities and dive survivability all while providing you a large mana pool to draw from.
If you plan on building Azure Core on ANY midlaner, you should complete the 1st mana stacking component for the item THEN finish whatever second or even third item you want before going back to finish Azure Core. This is more gold efficient overall and lets you stack your mana while having access to other core items you may need against the enemy team.
Megacosm is VERY important vs 1 or more tanks. You can change your build depending on the enemy team composition. If the enemy is on the squishy side, feel free to throw in Combustion instead of Spellbreaker. it synergizes well with your Q and can deal 50% of a Carry's HP with a single lvl 5 Q.
Caustica is a solid choice as it gives you mana regen AND more penetration.
I am also a fan of Dreambinder on Gadget but it isn't a necessity. Oblivion Crown or other damage items are also good, i just favor the control Dreambinder provides.
This build is tournament tested and i can confidently say it performs extremely well. Gadget requires good positioning and the ability to navigate a fight. So take it slow at 1st.