Book capital losses in gnucash
It depends on whether or not you are referring to realized or unrealized gains.  If the asset appreciation is realized, meaning you've sold the asset and actually collected liquidity from it, then Derek_6424246 has provided a good route to follow.   However, if the gains are unrealized, meaning only that the current value of the underlying asset(s) have increased or decreased, then you might want to record this under an Income:Unrealized Gains account.  One of the main distinctions between the two are whether or not you have a taxable event (realized) or just want to better track your net worth at a given time (unrealized).   For example, I generally track my retirement accounts increase in value sans interest, dividends and contributions, as income from an Income:Unrealized Gains account.  I can still reconcile it with my statements, and it shows an accurate picture for my net worth, but the money is not liquid nor taxed and is more for informational purposes than anything.   And no, I don't create an additional Expense account here to track losses.  Just think of Unrealized Gains as an income account where the balance will fluctuate up and down (and potentially even go negative) over time.