Edited Yukio Victoria (Sept. 1, 2014 12:34:51 AM)
118.9. Some triggered abilities are written, "Whenever gains life, . . . .“ Such abilities are treated as though they are written, ”Whenever a source causes to gain life, . . . .“ If a player gains 0 life, no life gain event has occurred, and these abilities won't trigger.
Example: A player controls Ajani's Pridemate, which reads ”Whenever you gain life, you may put a +1/+1 counter on Ajani's Pridemate," and two creatures with lifelink. The creatures with lifelink deal combat damage simultaneously. Ajani's Pridemate's ability triggers twice.