So just how is that O pronounced? This USn wouldn't pronounce it like, e.g., German grohtie.

I've always heard it pronounced /'grat:i:/. Are you thinking of the Val Girl groady (sp?) /'gro:di/ as in "Groady to the max; I'm sure; it's like totally nauseating; like barf out; gag me with a spoon!"? [Moon Unit Zappa in Frank's song Valley Girl] Google shows groady, groddy, and grody.