setting tildeop would be more like g~ but you can also pipe ranges of lines to tr(1) or rot13(6)/caesar(6) for things like gU:
:'a,'b!tr a-z A-Z
gu:
:'a,'b!tr a-z A-Z
and g?:
:'a,'b!rot13
:'a,'b!caesar 13
(though it's more challenging for things like word-wise or sentence-wise transformations)