Treat it like inventory, as its a cost of holding the asset.
You should be using the Inventory tab and recording each share market purchase as an inventory item, then when they are sold there will be an automatic transfer of the P&L. Rename the Inventory-on-hand account to “Stock Market - Acquisitions”
You create an Asset account called “Stock Holdings - Market Value Adjustment” which would sit just under the re-named “Stock Market - Acquisitions”
You also create an Equity account called “Stock Holdings - Revaluation”. Then you Journal the market movement between these two accounts.
Have you read this thread which contains a model of a Chart of Accounts set up.