If you find a transaction with a very high gain then look on the left side to see it's cost-basis. Chances are the cost-basis is really low.
This could mean that the market price that Koinly used for it when you bought this coin is wrong.
Click on the transaction to open up more information about it
Then click on the "eye" icon next to the "Sent" line. This will show all the transaction history for the coin.
Look at some of the oldest deposit transactions - does the market price look ok? If not, then edit it manually.
Koinly gets market prices from price aggregators like CoinMarketCap which usually do not have correct prices for ICOs or in the early days of a coin.
If the market prices are missing then you will see warnings like this on the transactions:
However, if the market price exists but it is wrong then you wont see any warnings and will need to spot them manually.
Sometimes a coin might appear on CMC with wildly fluctuating prices which means Koinly might end up using a price that is nowhere near what you bought the coin for. This only happens for assets with very low popularity or in the early days.