Google finance is a good platform to monitoring financial market.
The web site show a detailed information with news, performance and historical data.
There is a api tool for use it in google sheets, and i use it for monitoring my portfolio.
The integration with a google sheet is great, with a simple formula you can calculate a gain/ loss on single asset or total.
the free versione is not realtime but have 20 minutes of delay.
Unfortunately there isn't a built-in portfolio monitoring with stock hold, that is a problem if you want an alert or view quikly your gain/loss.
The only way is to use it with Google Sheet and write your formula for calculate a custom portfolio performance.