I subject accelerated in math, skipping 2nd. (That acceleration was reversed when we changed districts, so I did 3rd grade math twice. Ugh.) I didn't have any gaps - or if I did, I filled those holes in fast enough that they didn't bother me. I had to do several weeks' worth of "catch up" work following the skip, which was meant to prevent gaps, but really only made me hate subtraction.
We're only a week into DD7's skip (she was in 1st last year, and is now in 3rd). Her teacher says she's doing totally fine academically, and she has absolutely no concerns about her ability to do the work. Probably her biggest weakness is writing speed - she's had one page of "didn't finish in the allotted time, so needs to finish at home" homework, and on the spelling test today, she fell behind enough that she skipped one word entirely to catch up. (The teacher just handed her paper back and told her which word went in the blank.) Second biggest weakness is spelling for free writing assignments, but I haven't seen enough peer work to know if her spelling is average or below average.
In order for DD to get the grade skip, she had to pass a comprehensive evaluation demonstrating she'd mastered the 2nd grade basic standards. So while the kids who went through 2nd grade were exposed to more stuff (because the teachers introduce some 3rd grade concepts in 2nd), they weren't expected to actually know that more stuff. It's covered again in 3rd, where DD picks it up quickly enough to not fall behind. Because of that process, I don't anticipate any academic gaps.
Not what you asked, but I took 3 sets of college classes such that I was taking the first and second semesters of a two-term class concurrently. (So Intermediate Accounting I was the prereq for Intermediate Accounting II, but I took them at the same time.) If ever a situation existed where gaps would be a problem, you'd think that would be it. I never had any problem keeping up.
Also not what you asked, when my partner was in high school, she was "skipped" a grade. She was in a very low-income district with a high dropout rate, primarily due to parental pressure to work full-time to help support the household. In their school, they gave a grade-level achievement test (she thinks the Iowa Test of Basic Skills, but that doesn't cover the right grade levels) in 9th or 10th grade, and if your scores came back as at least a full grade higher than actual grade level, they gave you the option of skipping a year and graduating with 3 years of high school, rather than 4. Which sort of made my eyes bug out - scoring at 11th grade as a 9th grader on a 9th grade test doesn't mean you know what an 11th grader knows! But it meant that kids who otherwise would have been dropouts got high school diplomas, so had a better chance of breaking the poverty cycle.
Also not what you asked, people move from State 1 to State 2 all the time, without giving any thought to educational gaps. And I will say that EPGY math, which is tied to California standards, does not align at all with our state's curriculum standards. I could see a kid moving between states having significant gaps, even without a grade skip. But you never hear about that, now do you?
