Hey Matias,
Depends on a couple of things
How motivated / ambitious are you?
Gecko will certainly be trickier in the beginning and won't necessarily help develop the best habits but if you're happy to persist and spend a little more time in the water you can progress further with it.
Where do you sail?
If it's a bit warmer spending time in the water won't be a problem, if it's colder that tests the motivation a bit more. If you sail more in lighter winds the Viper with a centreboard will be much better. If you have regularly good wind the Gecko is more fun.
How often will you windsurf?
If you get out regularly (at least once every couple of weeks) your skills should improve quicker so the Gecko is more of an option, if you won't sail so often you'll have a better time with the Viper.
Best option if it's available would be to continue to rent / borrow a Viper while you develop your skills a little further before then purchasing something like a Gecko.
Key skills I would recommend before moving to this type of board would be
Comfortable lighter wind Tack and or Gybe
Using the harness
Beach start in water Knee to Ass depth
FYI there are a couple of different Viper sizes, here I'm talking about the 80, I used to run a school and used this board a lot for training people, great all rounder.
Hope this helps,
Dave