"Words neuter in Latin become masculine in Spanish"
This is generally correct!
In Latin, the most common type of masculine noun and the most common type of neuter noun look almost identical. They're only distinguishable in the nominative singular and the nominative/accusative plural.
In the branch of Romance that would eventually become Spanish, the nominative case disappeared altogether—so the neuter looked exactly like the masculine in the singular.
What's more, the neuter plural looked exactly like the feminine, for reasons that go back to Proto-Indo-European and the origin of the feminine gender in the first place!
So because of these very regular changes, the neuter gender vanished into oblivion, with its words merged into the masculine. Occasionally you'll find Romance words that are masculine in the singular but feminine in the plural, like Italian uovo "egg", uova "eggs"—these are the only surviving remnants of the Latin neuter. (In Spanish, this particular example got regularized by analogy, and is now a perfectly regular masculine: huevo, huevos.)
In the comments, you mention Latin dactylus (m) > French datte (f). This sort of change happens sometimes, especially in Gallo-Romance (which doesn't have the same distinctive masculine/feminine endings that Ibero-Romance does). But it's an irregularity, and not a very common one; the majority of words kept their original gender from Latin into Romance, and the ones that didn't can usually be explained by analogy.