The player can summon a Gaia Guardian by performing the Ritual of Gaia. This ritual is predicted to be mid/end-game to achieve, requiring the use of a Beacon, a Tier 1 Beacon base, 4 Gaia Pylons, and a Terrasteel Ingot.
When summoned, it shall rise above the beacon and quickly generate its health, similar to the Wither. Whilst doing this, the guardian shall be unable to move and shall stay shortly and temporarily paralyzed hovering on the Beacon (part of the ritual structure).
If the player wishes for a tougher challenge, with better rewards, they can summon a Gaia II Guardian using a Gaia Spirit Ingot, which requires you to defeat the original Gaia Guardian. The Gaia II Guardian has 450 health, higher damage, and has the additional effect of blasting the players with lightning sporadically.