To loop a Google Slides presentation, first set up a new slideshow in Google Slides as normal. When you're ready to run the slides on a loop, click the Slideshow button at the top-right to start playing your slides.
With the presentation view up, move your mouse to the bottom-left corner of the presentation screen to show the toolbar. On the toolbar, click the three-dot menu and expand the Auto-play item. Then on this menu, click the Loop option at the bottom to enable presentation looping.
Also on this menu, you'll need to choose the timing for your slides so that they play automatically. These range from short options like Every second or Every 2 seconds to Every 30 seconds or Every minute at the longest. Choose what timing is right based on your presentation type.