If you want to start gVim with tabs, the command-line switch is -p[n]. If you don’t specify n, the default is "1 tab per file you specify". If you specify n < w files, the files that don’t have tabs will be opened as buffers (see :help buffers for that; it’s a good topic for another post.) I’m not sure what happens for n > files; I assume it opens n tabs.

If you want to start gVim with windows, the command-line switch is -o[n]. It behaves like the tabs command-line switch.

For those that prefer examples (I know I do!):

gvim foo.txt bar.txt baz.txt

gVim opens with foo.txt in the primary buffer and the other files in other buffers.

gvim -p foo.txt bar.txt baz.txt

gVim opens with 3 tabs, one tab for each file.

gvim -p2 foo.txt bar.txt baz.txt

gVim opens with 2 tabs. foo.txt and bar.txt are in tabs, baz.txt is in a buffer.

gvim -o foo.txt bar.txt baz.txt

gVim opens with 3 windows, split who knows how.

The rest of the cases seem trivial to understand with these explanations.