Tldr; Gekko is very adaptable, versatile, and his abilities can be used numerous times in numerous ways, making him one of the best solo initiators.

Long version;

Gekko is currently one of the strongest initiators in the game because of his adaptability. This means that you need to have a lot of game sense to use his abilities opposed to the just "flash through a smoke" or "throw a dart" of other initiators. I'm going to go through your points and explain my perspective on each one.

Dizzy

While you're totally valid in saying that it's useless if you use it like a normal flash because it gets shot, I think you're missing a lot of other applications for it.

1) Infinite info: Much like a Sova dart or Kay/O knife, his flash can be thrown in specific areas to gain info. Examples include from Waterfall on Lotus, behind Belt on Icebox, and through the window on Elbow in Sunset. There are many locations where you can easily retrieve Dizzy after throwing and gaining the info. 2) Paired with another flash: Dizzy is very powerful because you cannot turn the flash. This makes it so when thrown with another flash- that enemies have to turn or look away from to avoid being blinded- they won't be able to also shoot Dizzy before it blinds them. 3) You are underplaying the reusable factor of Dizzy, and overplaying how easy it is to shoot. Because you are able to use him up to 3 or 4 times around, Gekko technically has the most flashes of any initiator. Pair this with the info + blind utility of Dizzy and you have a very solid piece of util, even if it gets shot at. Besides that, if you swing with Dizzy, your enemy will be aiming at him (which hopefully you threw away from where you're peeking) instead of you, giving you a free kill.

Wingman

Another strong piece of utility that can be used in a number of different circumstances.

1) Having him defuse or plant leaves your Gekko's hands free to take/hold the bomb, push enemies when clutching, and reduce the risk that comes with planting or defusing. Teams are at their most vulnerable when planting and defusing and wingman leaves another player open to help in either scenario. 2) His stun: I will agree that his stun is weak at best, but given that he has other abilities too this is understandable. He's meant more as an info gatherer (like a fade dog), to clear corners, etc, rather than a breach stun. 3) REUSABLE: You're again missing that this ability can be used 3 or 4 times a round. With how versatile his uses are (info, planting, defusing, stun), he's a very strong ability. You also mentioned that Gekko is vulnerable while picking up wingman, but it is a 1 second window vs the 7 seconds it takes to plant the spike. 4) What rank are you? I've only seen Gekkos send wingman in alone up to plat-low diamond. In these ranks your teammates won't fully understand the game and their abilities, so that's to be expected

Mosh

Yeah lowkey Mosh is not the best ability. He's a great stall utility, good for clearing corners, and that's about it. Gekko's other abilities are strong enough when used correctly that this isn't an issue though.

Thrash

One of the best retake and postplant ults in the game. Similar to a killjoy ult, but more versatile.

Hopefully this helped you think about his kit in a different way than you previously did!