The easiest way to convert Keynote files to Google Slides or PowerPoint is to use CloudConvert.
If you don't have keynote (or you don't want to install it) but you have a keynote file (.key) that you want to edit, the easiest way to convert it for use in PowerPoint or Google Slides is to convert it via cloudconvert.
Simply select the file you want to convert and 'start conversion'. You can then download the file and upload to Google Slides or open it in PowerPoint.
Another option is to use Mac OS's Preview to open the .key file and export it as a PDF. However, you will not be able to edit the presentation.