I wouldn't say that the criteria or system is that hard to navigate in Seattle. Those are both relatively fixed for many years. Basically, request to take the CogAT test in the fall and score at the 98% level.
What is a bit challenging is that the program has grown along with the rest of the district and there space/capacity issues that are being worked out. So you need to have a bit of tolerance for uncertainty until the final school locations are known. That said your son is young enough that at least the current growing pains will be over by the time he enters elementary school.
On the ground the APP program however has been really great so far for my 2nd grade son. The writing and science elements in particular seem fairly rigorous and it like other gifted programs has a nice cohort of kids.
Ben