Overview
Geki is a very weak character. In fact, he would be near unusable if it weren't for his ghosting. Geki relies on a very scummy way to take rounds, and even then, even taking a game isn't an easy time for him. His primary strength is if he gets a lifelead, he can start his ghosting, and as long as he keeps ghosting until the timer runs out, no one (except birdie...) can do much about it. Though Geki's gameplan is actually fairly simple, his significant weaknesses make being consistent with him extremely difficult.
General Gameplan
Geki, more than any of the cast, is an all-or-nothing character. The ideal strat with Geki is to get a lifelead and then ghost until the timer runs out. While it's a simple gameplan, and doesn't sound too hard on paper, there are many ways it can go wrong. The number one key to playing Geki is to not get hit. Geki's damage from his moves are very low in comparison to the rest of the cast. If you get hit, you're probably going to take 25% or more, and then you'll have to fight to get your lifelead back, and will probably take even more damage in the process. If you get the first hit, then start holding back and ghosting. Even when you're ghosting, you're still not out of the woods yet, as it'll be detailed below, but lifelead will be your chance to win the round.
What To Do Roundstart
Most of the action for Geki happens in the beginning of the round. Some good options on roundstart are his 'kara' claw (done by a microwalk forward and pressing HP), a jumpkick, or either of his shurikens. Your safest and most consistent option is to jump back, but your followup options after that tend to be telegraphed, so you shouldn't always jump back. While every option carries some risk, it all comes down to what you and your opponent does on roundstart. The one thing your opponent can do that tends to be dangerous is a forward jumpkick. If the jumpkick's slow enough, you can anti air with HP. If the jumpkick's too fast, your best option is to jump back. In the most dire of situations, you may need to teleport to avoid taking damage.
While Ghosting
For the most part, you'll only want to start ghosting once you get the lifelead. Sometimes however, you may be forced to go into teleport regardless of your predicament.
If You Have Lifelead
If you're teleporting while you have the lifelead, that's a good thing. You're on track to win the round, but the win isn't guaranteed yet. Your opponent's best bet in this scenario is to keep trying to switch directions to try to make you block the wrong way. Pay very close attention to which way the opponent's character is facing. Reacting usually isn't too difficult, but sometimes it gets weird. Don't panic, keep an eye on the opponent's movements. Some characters are far better at trying to trick you than others; you're a lot more likely to mess up VS. Eagle than Retsu.
If Neither Player Has A Lead
So you're not losing yet, but the situation isn't great for you. You'll basically be locked in teleporting as the opponent will be constantly chasing you. Finding a way to escape that pressure and/or landing a hit without getting one yourself won't be easy, and against some characters it's almost impossible. Against characters where you're safe on hit, find an opportunity to land a cheap shot on them, and it should work fine most of the time. Against characters where your stuff is often punishable, you'll need your opponent to slip up hard.
If The Opponent Has Lifelead
This situation's not too different from when you're at equal life, except this is tragically more common and way harder to feasibly escape. If you can get out of ghosting, do it. You might need to do something risky like a jumpkick to get your lead back. If you can't, however, there really isn't much you can do besides an HP and hoping for the best out of a teleport.