According to the Orbea website, the RS range extender battery cage is compatible with a "standard" size water bottle on the Rise M,H, URRUN & KEMEN series bikes.
RS Range Extender GEN 0
www.orbea.com
RS Range Extender GEN 1
www.orbea.com
It isn't clear about the X35 kit for the Orbea GAIN.
X35 Range Extender Kit
www.orbea.com
You can try downloading the user manual to see it any dimensions are listed.