Only dividends of Wahed FTSE USA Shariah ETF is subject to a 30% dividend withholding tax, but the capital gain is not taxed. These taxes are only for US-listed assets, regardless of whether the assets were obtained through Wahed Invest, or if you purchase it directly using your own broker. The dividends that you are entitled to receive for holding Wahed FTSE USA Shariah ETF are already net of these taxes.
For example, if your holdings of Wahed FTSE USA Shariah ETF has risen in price by 5%, that 5% is not taxed as it is a capital gain. However, if the ETF distributes dividends of USD0.10 per share, for example, the net dividend you will receive is around USD0.06 to USD0.07 per share after deducting 30% dividend withholding tax and other charges (e.g. foreign and service charges).