there is a way to run the webinar in just your browser without downloading anything by adding the following parameter to the end of the join hyperlink;
?clientType=html5
Here's an example of a sample session link with the parameter added: https://global.gotowebinar.com/join/123456789/123456789?clientType=html5. You can copy the link from either the confirmation email or the registration confirmation page.
adding the ?clientType=html5 at the end of the link will force it to run in your browser.