OK, i got that URL to open.
Try the following:
1. Create a new page.
1.1 Set the pages Content Container to Full Width:
https://docs.generatepress.com/article/content-container/
2. Add to the page a GenerateBlocks Container Block.
2.1 In the Blocks setting -> Sizing
2.1.1 set the Height to 100vh
2.1.2 set the Width: to 100%
2.2 In the Blocks Layouts set the Display: Flex, Flex-Direction column, Align Items Center, Justify Content Center.
2.3 In the Blocks Background -> Add a Background image.
3. Now add any blocks you want inside that Container Block.