Tomokazu Sugita is voicing the main mecha: Chamber.
Voice sample:
http://www.gargantia.jp/#mechanic_0
Ayumi Fujimura is voicing Striker, the other mech.
They announced a bunch of other voice cast for the show today too after the screening of ep1-2. There's a new PV on the site which is region blocked (#$^%#$), and they announced the OP/ED themes too.
OP: この世界は僕らを待っていた (This world was waiting for me) by Minori Chihara
ED: 空とキミのメッセージ (Message from you and the sky) by Choucho