You actually cannot convert gslides to pptx, i.e. convert Google Drive presentations to PowerPoint presentations.
However, when you open your .gslides link in Google Drive to access the Google Docs document, you can download the file in PPTX format using the File ► Download as function in the upper menu. This conversion is good when you want to continue to work with the presentation in Microsoft Powerpoint.
Updated: October 18, 2016