If I were to invest in Global ETFs using a US RoboAdvisor, will my capital gains be reduced due to tax or other factors once I decide to cash out?

This is a good question. For dividends distributed by U.S listed ETFs, they are subjected to a 30% witholding tax if you don't already know. When roboadvisors like StashAway or say any stock brokers receive the dividends on your behalf, they would also be taxed. When the dividends show up on your statements or in-app, it would have already been after tax.

Therefore, to some extent, your capital gains are affected.

You can find StashAway's official answer regarding taxes on dividends for US-listed securities HERE.​​​