See the bolded section directly out of the FAQ for the TV content recommendation. For gaming, experiment...
There are three offsets from reference level that you can select when the content has not been mixed to known standards:
0dB - Choose this setting for movies.
15dB - Choose this setting for rock music, pop music and other music with very highly compressed dynamic range (that is, there is little difference between the softest and loudest parts of the content. Think Metallica!).
10dB - Select this setting for Jazz etc where the dynamic range is wider and also for TV content as that is usually mixed at 10 dB below film reference.
5dB - Choose this setting for Classical music and any content that has a wide dynamic range.
Where do it mention gaming?