Other gains refers to any transaction that has been tagged with the 'Realized P&L' tag. This mostly applies to futures, options, contracts, and other leveraged activities.

This setting is only useful for people that are trading with margin/leverage/CFDs, When enabled, your other/margin gains are counted as capital gains and are included in your capital gains reports.

If this is disabled, your other/margin gains will not be included in your capital gains and you will be able to report them separately instead. For example, if your country requires gains from margins to be declared as Income then it is best to disable this setting.