Only fonts from Google Fonts are available in Google Slides. If you want to include any other font from Google Fonts, select the text and go to Fonts → More fonts… A new panel will open, where you can choose the font you want to use.
In PowerPoint, you can use any font installed on your system, regardless of its source. Remember that every time you change the original font, the layout might be affected and you may need to adjust it again.
To download any font from Google Fonts and be able to use it on your computer, click here to access and find the font on Google Fonts.
Then, click the “Download family” button, located in the upper right corner.
After downloading the font, install it on your computer. To do so, double-click on it and follow the steps. To make this new font appear in PowerPoint, close the program and open it again.