Effects don't create continuous effects.
611.2. A continuous effect may be generated by the resolution of a spell or ability.
Abilities create effects. Giant Growth has a spell ability that creates a continuous effect.
112.3a Spell abilities are abilities that are followed as instructions while an instant or sorcery spell is resolving. Any text on an instant or sorcery spell is a spell ability unless it’s an activated ability, a triggered ability, or a static ability that fits the criteria described in rule 112.6.
You seem to think "one-shot effect" means "spell ability", but that's not correct.
So, does the spell have a one-shot effect or does it create a continuous effect? It definitely creates a continuous effect.
610.1. A one-shot effect does something just once and doesn’t have a duration. Examples include dealing damage, destroying a permanent, putting a token onto the battlefield, and moving an object from one zone to another.
611.1. A continuous effect modifies characteristics of objects, modifies control of objects, or affects players or the rules of the game, for a fixed or indefinite period.