The toy sounds are already getting figured out by guys like cranejoe, only new form I heard about was Amazing Gummy

Gavv has been appointed an honorary member of the Japan Gummy Association. :lolol

Are these Granutes animal-based?

Definitely.

The first episode is out, and we learn that Gavv is half Granute.
With that, do you think his Rider form is also his Granute form given as the mouth parts for the monsters are also called Gavv’s?

Wonder how many riders will be in this season?

Gavv better “devil trigger”!

And I hate to see him hide his identity from everyone forever.

Shintaro Asanuma who voiced Zenkai Zyuran in Zenkaiger and recently Dragonalos in Gotchard will play the role of a new character named Kenzo Suga.

We're for sure getting three (Gaav, Valen, and a future pudding based Rider that is likely to be Sachika).

That's not really a confirmation but there's a shot in the opening where the trio sings the opening and you can see gavv in a bubble next to Shoma's face. and a similar bubble is present near Hanto (red bubble) and then Sachika (yellow bubble)

so yeah. The seeds are planted.

Also every Reiwa Heroine other than Mei have become Riders (well, Izu took until the movie but still).