Noun+型 - Grammar Discussion

model・type
pattern・kind
shape
size(d)

Structure

  • Noun + がた・けい
  • Noun1 + がた・けい + (の) + Noun2
  • Noun + の + かた
  • い-Adjective[] + がた

[型 is almost always read がた when paired with a noun due to rendaku]
[型 is used when referring to models, types or patterns (血液型), while 形 is used when referring to shapes (ハート形)]

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この星がたネックレスは金でできています。

Is there any reason I can’t/shouldn’t put の after がた? It wasn’t accepted when I tried.

Mostly because it’s a single noun, you’re not trying to specify what kind of necklace it is, as much as you’re just saying its a Star-shaped Necklace.