If you hold assets for business purposes, thier decline in value over time can be treated as a deduction for tax purposes, called Depreciation. This section discusses the handling of depreciation and appreciation of assets in GnuCash. It also provides a brief introduction to the related tax issues. Depreciation is usually only calculated on assets used for professional or business purposes, because governments don't generally allow you to claim depreciation deductions on personal assets, and it's pointless to bother with the procedure if it's not deductible. Capital Gains, which could be called asset value appreciation, are typically not recognized until the asset is sold, and at that instant, the entire gain becomes income.
Governments tend to be quite interested in taxing capital gains in one manner or another.
If you hold a bond that pays all of its interest at maturity, tax authorities often require that you recognize interest each year, and refuse this to be treated as a capital gain.