when I open a new document, it automatically has a gray border/grid/margin/frame/box/lines around it.
What’s your question? Those lines simply mark the area where your text will appear. You can hide them, but most experienced users would consider that as bad practice.
If menu View - Formatting Marks (the same that Ctrl+F10) is set, you will see a gray border; if Formatting Marks is unset, you will see at the corners something similar to crop marks.
To get rid of them, choose menu View and unselect Text Boundaries.
Tested with LibreOffice 6.4.7.2 (x86); OS: Windows 6.1.
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