Thank you for taking the time to view my guide. I will make this short and sweet. The first thing you have to know about Gaslight Joker is he is a beef castle. If built correctly, he is nearly unkillable. He is great at soaking up the damage and even dishing some out.

Laning Early Game: 
Ealry on, you want to focus on last hitting drones and keeping your lane opponent zoned out (harassing with Q and W as much as possible). If the potential for a kill is there, use your E to chase them down, fear them with your W, and smack them with your Q. If you are being ganked try to hit the laner and jungler with your W and pop your E for the speed boost (remember to use the security camera stollen power to see incoming ganks).

Mid Game:
During the mid game, you still want to focus on getting coins and last hitting drones (remember, coins and last hitting restore health). If you are able to get one or more of your lanes towers down, feel free to roam or assist the jungler. Surprise ganks for mid lane can really turn the tide of the game. Wait in the shadows for the opponent mid-laner to push, pop your E to move in quickly, fear them with W, turn your R (ultimate) on, and wail on them with Q and basic attacks (NOTE: Remember, your ultimate drains life on YOU, so do not leave it on if you are low on health). 

Late Game:
During the late game team fights are bound to happen. Be sure to be on the front lines of these fights. Your team is relying on you to be the tanky man wall that you are. Start off team fights by hitting as many opponents with your W as possible. When your team initiates, go in with your R on and pop your E. Remember to focus their squishiest champions first (i.e. Joker, Green Lantern, Gaslight Batman, Green Arrow, etc.). With their squishy/damage dealers down, push them back or flat out kill them. 

Mid to Late Game Notes:
Be mindful of when your team is doing global objectives like Leviathan. They will need you tank some damage and distract any potential steals.

Items:
As you can see, the core items are all health and health regeneration. The option items are for necessity. For example, if the team is building have Attack Damage, buy Fragment of Mogo. If they are building heavy Power Damage, buy Entropy Aegis. Soultaker Katana and Atlantean Seal depend on your play style (if you like basic attacking go with Soultaker Katana; if you like the bonus to your W and R, go for Atlantean Seal). If you want to be a straight tank, ignore the damage items and get both the Entropy Aegis and Fragment of Mogo.

I know this was short, but it should help you with starting to understand how GL Joker works (especially in top lane). Thank you for reading.