Gollum, Obsessed Stalker's last ability counts the total amount of life you gained without taking into account any life you lost during that turn. For example, if you gained 3 life and lost 3 life earlier in the turn, each opponent dealt combat damage this game by a creature named Gollum, Obsessed Stalker loses 3 life.

Gollum, Obsessed Stalker's last ability counts players who were dealt combat damage by any creature named Gollum, Obsessed Stalker. It doesn't matter who controlled those creatures.