Your stock’s Gain/Loss is calculated by subtracting the Current Value by the Cost Basis (the amount you have spent to own this stock minus any fees).
The Current Value can be found under the stock within your 'Investments' on your Portfolio page.
If you have only ever bought positions on a stock, the Cost Basis is simply the sum of all your Total Orders minus any FX Fees, if applicable.
However, if you have ever sold positions on a stock, the Cost Basis is calculated based on FIFO (first in, first out) cost queue. As this is calculated by the app, please ask the Customer Support team to supply the Cost Basis of your stock.