This question seems not to be written well. Questions such as this goes against certain aspects and guidelines mentioned in the OG.
1. In the thirty years since casino gambling has been legal beyond the state of Nevada, per capita wagering for residents of that state has risen markedly, though less than half as high as the amount in those states that received approval.
a. "those" states: There is no reference to the pronoun "those" in the sentence. While the gambling has extended beyond the state of Nevada, there is no mention of other states in the sentence.
The Official Guide for GMAT Review, 2015; Question No.: SC 89; Page: 688 - has a reference for this. When "those" does not have an antecedent, it is appropriate that the noun that has been referred is repeated.
Also, logically "those states" that received approval will also include "Nevada". The rate of rise in Nevada cannot be as half high as the rate of rise in Nevada itself.
b. "casino gambling has been legal beyond the state of Nevada" - is a present perfect continuous tense. The word "since" uses a past reference, and as per this sentence construct, there is no past reference as "has been" refers to an ongoing event from the past to the present.
A good grammatical sentence would appear something like this (per OG standard)
In the thirty years since casino gambling
was made legal beyond the state of Nevada, per capita wagering for residents of that state has risen markedly, though less than half as high as the amount in
any other states that received approval.