When choosing the print options in Gantter, you can choose to generate PDFs for the "Task List" and the "Gantt Chart" separately. This may help it to fit better on one page when printing.

Gantter generates the PDF as a single-page. The steps to break it up into multiple pages so the print is larger depends on your PDF software and your printer. This link can give you instructions to print as multiple pages using Adobe Reader: https://helpx.adobe.com/acrobat/kb/print-posters-banners-acrobat-reader.html

Another thing you can try is choosing the "HTML" print option from Gantter instead of PDF. You can then print the same as any web-page.