If the gap remains, twist the jump ring so that the two ends are not aligned.

Keeping one pair of pliers gripped on the jump ring, use the other pair to gently squeeze the rim of the ring bringing the ends closer together.

Once this is done, return the nose pliers to their original position on either side of the slit and twist until the two ends meet.

Be careful not to squeeze the rim too tightly or the jump ring will be disfigured.