The "Lion" is just some bogus name some author used for the Hanham Variation of the Philidor, going specifically through the Pirc move order to avoid certain problem lines after 1.e4 e5 2.Nf3 d6 3.d4 and there are problems with both 3...Nf6 and 3...Nd7. The Pirc move order was known before this stupid "Lion" name came about.
Not sure about 5.g4, but I know there are 2 Anti-Hanham lines if Black plays 3...Nbd7 (4.f4 or 4.g4) and one against 3...e5 (4.dxe5).
This line usually starts off as a Pirc, but transposes into a Philidor: 1.d4 d6 2.e4 Nf6 3.Nc3 e5 4.Nf3 Nbd7. According to the DB, it's the "Hanham, Lion Variation", which I never even heard of. Anyway, 5.g4 (instead of the more usual Bc4) is the Shirov Gambit. It's interesting, and I figure anything with Shirov's name on it should be a lot of fun.
I've tried the line in a few games with mixed results. I think this is one of the more interesting tries. I interposed 5.a4 and then g4, but that probably made no difference.