He thinks that our confused conversations about god, free will, freedom, causality, etc mostly stem from poor definitions of these terms. So to have a useful conversation and valid deductions, it's critical to have 'correct' definitions of these terms - including freedom, power, etc. He starts every section of Ethics with a list of definitions. He's not only making metaphysical claims, he's also trying to set humanity up for have better conversations and understandings about god, society, and ethical behavior (hence the title of the book "Ethics").

Here's a good quote, wherein he's criticizing people's conception of God as the 'first cause' -

    "So they will not cease to ask for the causes of causes, until you take refuge in the will of God, that is, the asylum of ignorance."

The quote makes pretty clear that he is explicitly rejecting the "God of the gaps".