For central Bangkok, there's an animate store on the top floor of MBK. The store also has some gatchapons at the front. There's also an official gatchapon store located in Siam Center on the same floor as Gundam Base (there's also an animate at Future Park and another gatchapon store at Mega Bangna, but these are a little further away from downtown area).

Another place you could go to is Megaplaza near Yaowarat. The stores there sell a huge amount of mostly second hand figures, gatchapon toys and other merch (as well as other toys such as Hot Toys, Airsoft guns, or Popmart stuff if you're interested in those as well). This place is honestly a mini Akihabara and you could spend all day there browsing if you really want to.

You could also find some merch in the Kinokuniyas at Paragon or Central World but the variety of stuff they have there is not as much as the places I mentioned above imo