It's 3 weeks since Gekko released, here are some tips on using his flash:

• Although we should all intuitively understand this, I do not see players taking advantage of the "kill zone" or "flash zone" (depending on what coaches/players you listen to) concept.
  • A flash/kill zone (I will use flash zone from now on) is an area in which you would be heavily punished by a flash. This is classified by open areas lacking cover. The best place to use a flash is a place where somebody cannot avoid the flash and where they have nowhere to hide.
• The best part of Gekko's flash is that it can be quite difficult to shoot it at times. Therefore, by playing around flash zones, which is easier to do on defense, and putting in a little time to prepare a good flash in a custom game, you can put yourself leagues beyond the competition.
• As for when you're on offense, I suggest thinking of your flash as a mobile Fade Haunt, but instead of revealing people, it blinds them. On offense, your primary objective with Dizzy should be information gathering. Yes, the fact that Dizzy flashes your enemies is nice, but any other debuff would be just as effective. Every part of Gekko's kit is perfectly made to be played off of by your team, be sure to use his kit as such.
  • Dizzy is like Skye's flash at times. If it doesn't shoot, you know that area is clear! Alternatively, you must mentally model what isn't cleared by Dizzy, and be ready for enemies in those areas.
• On top of optimizing Dizzy for its literal usage, you must, more importantly, optimize Dizzy for repeat usage. A Gekko player who cannot take advantage of the 10 second cooldown is destined to be limited in their play.
  • On defense, low rooftops and open windows are your best friends. Think Art on Pearl, B Main on Ascent, Belt on Icebox (the rare attacker example!), and C site on Haven.
  • On offense, there are some rare examples of the same concept, such as Belt on Icebox, however, your main optimization will come by using Dizzy more conservatively than you may be tempted to. Throwing Dizzy deep into an area may actually be a net negative for your team; although you may have gotten slightly more information, Dizzy's Globule (the orb thing) is now difficult to retrieve, which limits your team's potential to rotate and quickly pivot based off of any changes in circumstances.

TLDR:

• Prioritize the value of quick cooldowns over increased single-use value
• Pay attention to all the information Dizzy provides you (what it did and did not clear)

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If you want to read more about this, I made a pretty lengthy post about it here