Here is the Mecha Animes that I know
1. Tranformers Cybertron
Transoformers Cybertron was one of the best Mecha Animes I seen there are 52 eps of It and kept me going the whole of 2007. There theme song for Tranformers Cybertron is very cool and the effects are very awesome unlike that one show they show on TV called Are you afraid of the dark.
I don't know much about Gundam but I know that it's one of the most longest running Anime series ever.
Voltron is a 60's mecha series I don't know if it's Anime because I seen i about 4 years back. Voltron was very boring....only a few of the eps were good.
4. Transformers Energon
Tranformers Energon is another cool Mecha Anime show. It's part of the Unicron series aswell as Cybertron. The characters here have Power link like two transformers can combind to become one. In Cybertron only Optimus Prime and Leobreaker can power link but the tranformers don't know about power linking in Cybertron.
Those are some of the Mecha Anime I seen over the past 7 years.