On console you do not have a choice to make. PS5 and Xbox Series X|S both use fast internal solid-state storage, and a game installed on either one is already running from an SSD. The question only becomes interesting if you are thinking about a PC version - and there is not one announced.
Why the storage matters for this game specifically
Leonida streams as one continuous world. The showcase footage moves from street into shop interiors and back out without a loading break, and drives between visually distinct regions without a pause. That behaviour is built directly on the assumption that the machine can pull assets off storage faster than the player can move through them.
GTA V faded to black when you entered a building because it launched on the Xbox 360 and PS3, where a mechanical hard drive could not keep up. Removing those fades is not a stylistic choice; it is what the hardware now permits.
Storage rules per console
- PS5 and PS5 Pro: the game must live on the internal drive or a compatible internal M.2 SSD. A USB external drive can archive it but cannot run it.
- Xbox Series X|S: the same restriction - internal storage or an official expansion card.
If you are short on space, moving other games to external storage and keeping GTA 6 internal is the correct arrangement.
What about PC?
Rockstar has not announced a PC version, which means no system requirements exist and cannot exist. When one is announced, an SSD will almost certainly be listed as a minimum rather than a recommendation - that has become standard for games built around continuous streaming. Until then, any requirements list you find online is fabricated. Our system requirements page explains why in detail.
Install size guidance is in our storage answer.