The solutions depends on
a) do you want to reflect just the Investment re-valuation - the movement in the paper value, or
b) do you want to reflect the “unrealised” gain/loss as Income
The recommendation would be (a), otherwise if you use your P&L for tax return purposes (actual gains/losses), then the inclusion of unrealised transactions could cause you problems.
To do (a), under BS Assets create an “Investment Holdings - Market Value” account and under BS Equity create an “Investment Holdings - Revaluation” account. Then via Journal you would post the month end unrealised gain/loss between these two accounts. At each month end you would edit the previous month journal by changing the date and the new revaluation amount
The Assets would look like this
The Equity would look like this
This way only actual gains & losses are reflected in the P&L and the paper gains & losses are reflected in the BS