Gerund Phrases after adjectives

In this sentence, "It's difficult being an architect", is "being an architect" a gerund phrase and, if so, is it the subject?

You can call it a subject, if you want to. It behaves like a subject - verb agreement, word order, undergoes Raising, etc. But it has no meaning, and it's a dummy, inserted by the syntactic transformation Extraposition.

You can also call the extraposed clause being an architect a subject, if you want to. It's meaningful, unlike it, and it also appears as subject in the non-extraposed sentence

• Being an architect is difficult.

which means the same thing.

That's because the transformation changes the subject, but doesn't change the meaning. Syntactic transformations like Extraposition or Passive don't change meanings. The various jobs of subject are parcelled out among two constituents in this sentence, and neither has all of them. So which one should get the label isn't clear, and that's because the label itself isn't clear -- "subject" is a term invented by ancient grammarians, not something you can discover scientifically.