These are missing and might be unnecessary. I am not confident or motivated enough to edit this into the actual page.
Vertex - They haven't put out much yet but they seem pretty good.
Orca Toys - They mostly make girls in swimsuits or lingerie in a very wide variety of scales.
Flare - Creates retail PVC versions of popular garage kits such as those from the hit sculptor @frenchdoll1
One anon once posted "They're essentially the remains of Arcadia's pvc figure production with the same ownership as Alter Almost all their sculpts come from VERY highly regarded garage kit circles."
Plum - They mostly produce Ro-Kyu-Bu! and Yoshino. They recently had major production issues with their latest Yoshino so I'd hold off on pre-ordering new sculpts they've shown that they've gotten their shit together.
Gift - Popular for their production of plush characters. They make soft Touhou girls.
Aquamarine - a manufacturer that uses GSC as a distributor. Makes 1/7 and 1/8 sculpts of characters from a wide variery of franchises.
Pulchra - They have an awful history with scales that have turned out hilariously bad. Wait for reviews of the final product. Aside from the standard PVC that comprises their small collection of releases they have also worked with resin and cold cast.
Stronger - An offshoot of GSC, started by a couple of their sculptors who were initially contracted to do Aniplex-exclusive meguca. Since then they've started doing other exclusive for other production companies. You will likely have to use a forwarder or store that deals in exclusives to obtain their items.