To clarify:
- building V8 never requires internet access
- getting the V8 source and build dependencies always requires internet access (how else would you download anything?)
- GYP is not part of depot_tools
- GYP is always required for building V8
- GYP does not require internet access
- there's a reason none of our documentation mentions "git clone ..." directly: that's not how things work with V8
So what you'd do is:
(1) on a machine with internet access, install depot_tools and put them in your $PATH
(2) on that same machine, "fetch v8"
(3) tar up the result and transfer it to the other machine without internet access
(4) build ("make x64.release" or whatever target you're interested in).
Then again building from source on a machine without internet access makes me think that you're not planning to update very often. That's a bad idea, so part of me hopes that you'll abandon the entire plan in favor of something more sensible.