Is the phrase “God is love” a metaphor?
The problem you pose is a good one, but it is hard to explain because of the way languages work.
In the English language, grammatically and secularly speaking, Love is an abstract noun, which is a noun denoting an idea, quality, or state rather than a concrete object (concrete = physical object). Spiritual terms will often be abstract nouns, since they describe a condition or state of being, such as “good”, “bad”, etc…
Given that Jesus - in the flesh - is one in being with the Father and the Holy Spirit, however, we have a problem with the language; whereby a physical object (Christ’s body) becomes basically the same as the abstract, spiritual state of “God as Love”.
Additionally, in Catholic terminology, the Love between God the Father and Christ the Son is so strong it takes on a life of it’s own, which forms the third person of the Trinity, who we call the Holy Spirit, the Paraclete.
Returning to the English language, we can “work around” the notion of Love being an abstract (slightly metaphoric or spiritual) concept by relating it to Jesus’ name, since a name is a personal pronoun. When God is given a name, as such, the noun takes on a more identifable, concrete state (in the English language).
That said, when we Catholically say “God is Love” in the English language, we relate the abstract noun “Love” to the personal pronoun “Jesus”, His Father and the Paraclete all at once; and, therefore, the meaning is altogether abstract, personal and concrete.