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.

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.