Do acoustic guitar gig bags protect guitars from humidity?

Generally speaking, gig bags do not offer guaranteed protection from humidity. Gig Bags are generally not air-tight, which makes controlling the levels of humidity challenging.

Are all acoustic guitar gig bags good for acoustic guitars?

No. Acoustic guitar gig bags come in a variety of sizes to suit various shapes of acoustic guitar. For example, a jumbo acoustic gig bag would be inappropriate for a classical acoustic guitar.