I’m working really hard on memorizing the circle of fifth, triads, 251 progressions and everything that is mentioned in the most basic course.
II’m also working on “tune up”, and I feel that it’s time to move on to the next course but I’m far from memorizing all of the things that I’ve mentioned above. It might take a few months until I’ll remember everything. So what is the right approach? Should I move on to the next course knowing that I’ll have gaps or lack of knowledge in the materials?
Hi Omer,
I’d keep revisiting what you’ve done, but add to it with the next lesson(s). Otherwise, I think you’ll get frustrated–and worse, just plain bored. I’ve been here for about a year and a half. I still go back over the foundation courses from time to time just to make everything more clearly embedded, more automatic. As Hayden has said many times, coming to grips with the keyboard and theory is a life-long endeavor. Have fun. 
Hi Omer 
Yes I certainly recommend moving onto the subsequent courses.
Our Beginner Jazz Syllabus contains 2 courses to be studies in tandem which gives a nice mix of theory and jazz standards.
Once you have completed the foundations course and perhaps experimented with some of the beginner jazz arrangements, I’d recommend diving straight into our course on chord extensions:
Just as @scott1 mentions, we can always revisit the earlier theory. It’s good to do so and this doesn’t need to stop us from ‘getting our toes wet’ in the more advanced theory.
My view is that it’s always good to expose ourselves to more advanced theory, even if we don’t understand it at the time.
I recommend to alternate the practice plans that you can find here:
Perhaps spend 2 days on the foundation plan, then 1 day on the chord extensions plan, then repeat. That way you are covering the basics whilst also stretching your knowledge in new areas. You can adjust the ratio as you progress on through the courses.
Finally, set a target with learning new tunes. I can’t stress the importance of this enough.
Aim to have a repertoire of 20 tunes within the next 12 months. You will forget some of them which can be frustrating, but as jazz musicians we will be playing these tunes for the rest of our lives and after many years of playing the tunes the memorisation becomes easier and easier.
As Scott mentions, learning jazz is a life long endeavour and there is always more to learn. A sizeable chunk of jazz theory is broken down in our practice planners for the first 4 jazz courses: PDF Practice Plans - PianoGroove Community - there are some useful comments and guidance in those threads which are worth reading over.
After completing these 4 courses, and learning 20 standards, I guarantee that you will have gained a new found freedom with the piano and with your harmonic knowledge.
The syllabus pages are also useful to get a ‘birds eye view’ of the subject at hand so it might be nice to skim through them to see the journey ahead:
https://www.pianogroove.com/syllabuses/
Above anything else try to enjoy it Omer. With such a vast subject it can be overwhelming at times but just chip away at the task each day and you will see rapid improvement with the PianoGroove system.