As its name implies, it was apparently a Summoning Gate once wielded by the Guardian of Time and has a Fire Elemental Attribute
In addition to Lana, Cia can use her Scepter to create Summoning Gates that resemble the Guardian's Gate in order to summon Dark Links. The Guardian's Gate also featured in the design of Cia's Level 2 Scepter Guardian's Scepter.
Lana (Cia recolor outfit) wielding the Guardian's Gate from Hyrule Warriors
The Guardian's Scepter with Guardian's Gate incorporated into its design from Hyrule Warriors