Reverse A Group Of Lines
The following command can be used to reverse the order of all lines in a file by doing a global move on all lines that match the beginning of line character starting at the theoretical 0th character:
:g/^/m 0
Reversing a range of lines is a little more work. Just as the previous example needs to be anchored against the 0th character, a specific range of lines needs to be anchored at the line just before the range. Thus reversing the lines 5 to 10 requires being anchored at line 4, like so:
:4,10g/^/m 4
See :h 12.4 for more details on how this works.