The Black Gate

As The Black Gate enters, you may pay 3 life. If you don't, it enters tapped.
{T}: Add {B}.
{1}{B}, {T}: Choose a player with the most life or tied for most life. Target creature can't be blocked by creatures that player controls this turn.

Tales of Middle-earth Commander

As The Black Gate enters the battlefield, you may pay 3 life. If you don't, it enters the battlefield tapped.
{T}: Add {B}.
{1}{B}, {T}: Choose a player with the most life or tied for most life. Target creature can't be blocked by creatures that player controls this turn.

The chosen player needs only to have the most life or tied for most life as The Black Gate's last ability resolves. After the ability resolves, any changes in life total won't affect which creatures can block or be blocked that turn.

The creature that was targeted by The Black Gate's last ability can't be blocked by any creature the chosen player controls, including creatures that weren't on the battlefield when The Black Gate's ability resolved.