Originally called Main Gate, it was renamed in honour of Elizabeth II in 2012 to mark her Diamond Jubilee.
From Historic England entry:
“ROYAL BOTANIC GARDENS KEW Principal entrance gates and railings fronting Kew Green
(Formerly listed as Principal entrance gates and railings fronting Kew Green, ROYAL BOTANIC GARDENS)
II* Gates by Decimus Burton 1848. Ironwork by Walker of Rotherham. Foliated and scrolled design with armorial centre-piece, set in mitred framework with two rows of anthemion pattern dog-rail. Elaborate overthrow. Matching single side entrance gates, all between Portland stone piers with garlanded panels by John Heming Jnr. The central piers are crowned by vases which came from an earlier pair of entrance gates to Kew Palace. Spear-headed railings with smaller, closely-set spearheaded dog-rail forming quadrant to either side of entrance and terminating in coupled, fused, Portland stone piers with urns.”