There is Godzilla 1954-84. Those all are in the same universe. Then 1985 to 1999 minus the USA one is another continuality.
Not quite. The first continuity, the Showa era, ended in 1977 with Terror of Mechagodzilla
. The second, Heisei-era continuity began with the 1984 The Return of Godzilla
, though that was released in the US as Godzilla 1985
. It disregards every previous Godzilla film except the original, claiming to be Godzilla's first attack in 30 years. The Heisei series concluded with Godzilla vs. Destoroyah
Then the ones ending in Final Wars ignore everything besides themselves, 1954 and dimes 98.
The six Millennium-era films comprise five distinct continuities, since the fourth and fifth (the Kiryu/Mechagodzilla films) were in the same universe. The first three are standalones, acknowledging nothing but the '54 original -- although, as I said, GMK does reference the '98 US film briefly and the two can be seen as sharing a common continuity. The Kiryu films acknowledge the '54 film (with a slightly changed ending) plus several of the non-Godzilla Showa-era films including Mothra
. Final Wars
also acknowledges Mothra
and several other Showa films. This is discussed more in the link in my above post.