It tells you if you have the SNP it tests for in those genes. It doesn't tell you anything else.
Gilbert's syndrome is pretty common. The vast majority of people that have it are not bothered at all. There is also no proof that lifestyle interventions are in any way helpful. This is the case for basically everything that 23andMe tests for.
If you want to raise a proper counterexample, you would choose the BRCA1 gene. But again 23andMe's test doesn't tell you whether or not you have a BRCA1 mutation, it only tells you if have one of the SNPs they test for. If you don't get a positive result, and that is the vast majority of people, you aren't any better off. That's why in the clinic when someone needs a BRCA1 test, we sequence the whole gene using next generation sequencing technologies.
The majority of people are bothered by it. Having a 1/3 of the ability to detoxify certain things means stresses on other systems. Very few are saying my GS is making me feel foggy and tired but it does.
There are a lot of studies that show lifestyle influencing Gilbert Syndrome sufferers. The most common suggestion is to avoid drinking.
Not sure it's as completely useless as you believe.
UGT1A1 UGT1A10
You can take that information research more or change your lifestyle / diet to put less stress in processes that involve those genes.
Very useful or useless depending on who you are.