We have designed several GFRC modular planters. More because of the need for a seamless, double-curving facade than easy modular construction. Ours was designed to be attached to a steel framework surrounding a concrete walled waterproofed planter.
Personally I prefer concrete or stone as they wear better and ‘feel’ more durable. GFRC depends too much on the painted finish. It is very light weight though…
looking at your sketch I am guessing you are trying to achieve a prefab planter wall but I guess I lack enough context to have a feel for whether it works or looks ‘good’. It does seem kind of large and unwieldy for what you get.
Out of curiousity, why not just use lightweight concrete or brick with a rendered/steel/wood facade?