Hi Julie,
– How do I remove the menu on a landing page on both desktop and mobile version ?
1. Go to appearance > elements, and activate the Disable element module.
2. Go to appearance > elements, create a new layout element.
https://docs.generatepress.com/article/layout-element-overview/
3. Tick the Site header, mobile header, primary navigation under the disable elements tab.
4. Choose front page as location under the display rules tab, publish the element.
– How do I remove site tittle or/and logo (I’m not sure yet what I will choose) ?
Is it site-wide? If so, go to customizer > site identity, you can remove/upload/update logo and site title there.