The mooncakes made in Guangzhou taste good

完整句子是：
The mooncakes which are made in Guangzhou taste good

which are made in Guangzhou 修饰月饼，缩写后省略which are

把句子拆开了。 made in Guangzhou 是直接形容 mooncakes.
原来的句子 The mooncakes taste good 已经是一个完整的句子了。
习惯用法。

made in Guangzhou 作为The mooncakes 的定语过去分词作为定语

The mooncakes taste good是主干taste是谓语。

口语里简化了

二楼正解