How can I indent multiple lines, and indent "backwards", in gvim?

Start at the first line you want to indent, then press > and type the number of lines you want to indent and press > again (for 10 lines you'd press >10>) To un-indent you'd just use < instead of > (<10<)

What you want are the > and < commands, see ":help shift-left-right".

You can use these commands in multiple ways, but since you specifically mentioned highlighting, you can just use your mouse or keyboard to highlight the lines you want to shift/unshift and press > or <.

Instead of using visual mode (highlight) you can provide a count and >> or <<. For example, 3>> will indent the current line and two lines below it.