ParseGID = function (gid)
{
    var flippedHorizontal = Boolean(gid & FLIPPED_HORIZONTAL);
    var flippedVertical = Boolean(gid & FLIPPED_VERTICAL);
    var flippedAntiDiagonal = Boolean(gid & FLIPPED_ANTI_DIAGONAL);
    gid = gid & ~(FLIPPED_HORIZONTAL | FLIPPED_VERTICAL | FLIPPED_ANTI_DIAGONAL);

    // Parse the flip flags into something Phaser can use
    var rotation = 0;
    var flipped = false;

    if (flippedHorizontal && flippedVertical && flippedAntiDiagonal)
    {
        rotation = Math.PI / 2;
        flipped = true;
    }
    else if (flippedHorizontal && flippedVertical && !flippedAntiDiagonal)
    {
        rotation = Math.PI;
        flipped = false;
    }
    else if (flippedHorizontal && !flippedVertical && flippedAntiDiagonal)
    {
        rotation = Math.PI / 2;
        flipped = false;
    }
    else if (flippedHorizontal && !flippedVertical && !flippedAntiDiagonal)
    {
        rotation = 0;
        flipped = true;
    }
    else if (!flippedHorizontal && flippedVertical && flippedAntiDiagonal)
    {
        rotation = 3 * Math.PI / 2;
        flipped = false;
    }
    else if (!flippedHorizontal && flippedVertical && !flippedAntiDiagonal)
    {
        rotation = Math.PI;
        flipped = true;
    }
    else if (!flippedHorizontal && !flippedVertical && flippedAntiDiagonal)
    {
        rotation = 3 * Math.PI / 2;
        flipped = true;
    }
    else if (!flippedHorizontal && !flippedVertical && !flippedAntiDiagonal)
    {
        rotation = 0;
        flipped = false;
    }

    return {
        gid: gid,
        flippedHorizontal: flippedHorizontal,
        flippedVertical: flippedVertical,
        flippedAntiDiagonal: flippedAntiDiagonal,
        rotation: rotation,
        flipped: flipped
    };
}
See Tiled documentation on tile flipping:
http://docs.mapeditor.org/en/latest/reference/tmx-map-format/

@function Phaser.Tilemaps.Parsers.Tiled.ParseGID
@since 3.0.0

@param {number} gid - A Tiled GID.

@return {Phaser.Types.Tilemaps.GIDData} The GID Data.