It may seem obvious that spells are needed to perform magic, but that depends on our definition of “spell” and “magic.” Magic is considered powers that don’t exist on Earth. But in our fictional world, magic is real, so does this definition fail? On that world, yes, but our audience is on Earth, so it still holds. But any physical changes that can be done without manually manipulating something, or using tech, can be considered magic. As for spells, this typically means words, gestures, physical materials, or some combination of these, to perform said magic. If all of this seems obvious, there’s a reason the distinction is being made.
Gods are considered real in fantasy worlds especially. And gods make things happen that don’t occur in the real world of Earth. Does that mean they’re doing magic? Unless we have a pressing reason for using a different word, yes. Perhaps gods and mortals are tapping into the same well of supernatural power, but one difference is that people are weakened by this and gods either aren’t, or to a lesser degree, but we can safely say gods (usually all of them) are capable of magic.
But are gods using spells? Or are they doing magic by force of will? I’m inclined to say it’s will power for several reasons. One is that we should distinguish between gods and mortals. It seems clear that gods can innately do magic (will power), but mortals must struggle to learn and master it, even if they have the talent from birth. Gods may have imposed limits, via spells, on what mortals can do. We can find other ways to impose limits, but it still raises the point: either beings can do magic by force of will or they need spells.
We’ve seen depictions of gods making gestures or speaking words to achieve their result, which suggests they’re using a spell, but perhaps they’re just controlling the force better this way. In visual media, people are typically shown doing these as visual/audible cues as to who is causing something to happen. This is sometimes omitted when it’s already been established who in the scene has the ability, such as Darth Vader in Star Wars, or when ambiguity is desired (either by characters or storytellers). Since will power is innate, gods may not need spells. Non-gods are typically portrayed as needing to learn how magic works; they study various things like the language of magic (an innate skill for gods), and then read spell books to learn how each spell is done. Then they practice. What little power they start with grows with that practice. We assume gods aren’t doing any of this because we show them as strong. Whether gods are using spells or not is semantics, except that if we have a choice between gods and mortals using willpower, the least sensible option is gods needing spells and mortals using willpower, because the latter suggests fewer restrictions and more power, both of which apply to gods.
What about mortals? Can they do magic via will power or are spells the only way? We may want to have both in our setting and use a name for each, like wizard vs. sorcerer.