At least 5-6 live-action series, 2+ movies, and 2 anime series. Series 1 and 2 focus on Kouga Saejima, the first GARO of the franchise. Series 1 is the world setup, and series 2 'Makai Senki' adds to it, with a schism occurring in the Makai Order. Series 3 on the other hand featured a really annoying lead, Ryuga Dogai (Think Japanese Mentos Commercial protagonist with the GARO Armor). Series 4, on the other hand, returns to the Saejima lineage, with Raiga Saejima, the son of the first 2 series's mains (Kouga and Kaoru). Series 5, 'GOLDSTORM', returned to Dogai, and featured a new villain (Played by Kamen Rider Decade himself- Masahiro Inoue).
Vic Mignogna confirmed that he's done some recording for the first series. We don't know which role yet, though.
You probably don't see it, but I am doing a happy dance similar to Godzilla's from 'Monster Zero'. Also, I keep seeing Eric Vale as either Prince Alphonse or Leon Luis, and Christopher Sabat as Herman (I still don't know why).
Update- So, this series is getting a dub. Glad this is happening. Wondering who the cast will be.
Actually, the guy who wrote the script/ story was a little-known character/ creature designer and director named Keita Amemiya. GARO was one of his works, with the original series having debuted in 2005.