How to indent multi-line in vim/gvim?

Yes. Try:

V             # To switch to visual mode. Select lines, and...
>             # Indent (use `<` to de-indent, or with a number just before to indent several times).

The indent commands =, < and > can be combined with all movement commands and text objects. For example:

>G     Indent until end of file
>}     Indent until next paragraph
>iB    Indent contents of current { } block

They also indent text selected in visual mode.

Use < to un-indent or = to re-indent ('format' using the filetype settings).