To Remove Sidebar in GeneratePress Theme, just follow these steps
Remove Sidebar in Specific Post & Page
Step 1. Edit Page or Post you want to Remove Sidebar

Step 2. Scroll down
If you want to remove:
- Left Sidebar > Check on Content/Sidebar
- Right Sidebar > Check on Sidebar/Content
- Both Sidebar (Left & Right) > Check on Content (no sidebars)

Step 3. Click on Publish (or Save)

If you don’t see Layout Section. Just click on “three dots” > Options

Then check on Layout

WordPress will automatically reload the page and save the settings!
Remove Sidebar in WordPress Post & Page, Post Categories/Tags
Step 1. Appearance > Customize

Step 2. Layout

Step 3. Sidebar

Step 4. Remove Sidebar
With GeneratePress Theme (Free), you can remove Sidebar at:
- Posts (Pages) > Check on Content (no sidebars) at Single Post Sidebar Layout

- Post Categories/Post Tags > Check on Content (no sidebars) at Blog Sidebar Layout
Done!