There was an original study that showed that people with GS had 31% slower processing of acetaminophen, so it was thought to be processed by UGT1A1, the enzyme deficient in GS. The advice given was for people with GS to avoid acetaminophen. Later experiments showed that acetaminophen was actually processed by UGT1A7, and so acetaminophin was given the green light for people with GS. Later still, it was shown that most people with the UGT1A1 mutation ALSO had an even worse mutation in UGT1A7. This explained the results of both the previous studies, and justified the results of the original experiment - that people with GS should avoid acetaminophen (Tylenol). 

 Since we have problems metabolizing it, the effect is that it sticks around in our bodies a lot longer than most people. Since acetaminophen can cause liver damage, it seems best to avoid this.