Checklist of Toyota parts needed for grille replacement from Matt's detailed thread:
53149-48050 (Gasket, Grille) - most expensive part if missing
90467-10190 (Grille plastic center clip that locates it) - Matt's wasn't broken. Mine's missing.
90109-05092 x2 (Stud, Bottom M5 - 0.8) - This part number is for a Lexus ES; on the RX, Lexus only offers them as a package with a brand new grille. You also need these for the lower tabs to adapt a 330 grille to a newer RX if your new grille doesn't come with it.
90109-05103 x3 (Bolt, Top M8 - 1.25 in, M5 - 0.8 out) - Top larger stud/nut combo that hold the grille in. These can be substituted for a 10mm or 12mm long M8-1.25 bolt, or can be screwed into the new grille while saving the old nuts. This is also an ES part.
90080-17202 x5 (Nut M5 - 0.8) - Nuts that hold the grille to the hood.

Felix said he also installed washers under the nuts.

 

It's not essential but for how cheap it is I'd grab one. It really helps make the installation a breeze. It holds the grille in place which makes tightening the bolts a lot less finicky.

 

Thats what I find.......

 

So it turns out that the grille ( ) came with gasket and the clip! Just need the bolts and nuts. There is no extra space for a washer with the original bolts and nuts.
Super easy to install. The gasket was already installed on the grille. It is a harder plastic than the original gasket, and is slightly visible around the edge, but I'm happy with it for the price and ease of installation.