The dashboard includes all transactions imported into CoinTracking, while the gains page does not include deposits and withdrawals.
Deposits and withdrawals should be a closed loop for internal transfers of coins from one of your wallets / exchanges to another.
If you only have one (deposit or withdrawal), the balance on the dashboard will be incorrect, because you always need to match a withdrawal with a deposit.
The Missing Transactions Report shows where deposits and withdrawals are missing.
For more information on missing transactions, see this article: Missing Transactions Report: White entries (no match)