There are 2 methods convert PowerPoint for use in Google slides.
Method 1 bring the complete presentation into google slides or method 2 convert a slide or group.
Method 1 Complete Presentation
1. Open Google Drive.
2. Select “New” in the upper left-hand corner of the screen.
3. Select “Upload File.”
4. Select the desired PowerPoint presentation.
5. After uploading, right click and select “Open with,” then select “Google Slides.”
6. Select “File.”
7. Select “Save as Google Slides.”
Method 2 Converting a range of PowerPoint slides
1. Go to Google Drive.
2. Select Slides.
3. Open a “Blank” presentation.
4. In a new Presentation, select “File,” then “Import Slides.”
5. Upload the PowerPoint file.
6. Select the newly-converted Slides.