A player told me that attacking a planeswalker with a creature with lifelink doesn't provide lifegain. It sounds strange to me since damage are dealt by the creature (whatever to who, what).
This player was a judge but since he was also my oponent, I query about this point of rule...
You will gain life by attacking a planeswalker with a creature that has lifelink. More generally, any damage source with lifelink will cause you to gain life if it deals damage to anything.
The definition of lifelink in rule 702.15b says this:
Damage dealt by a source with lifelink causes that source’s controller, or its owner if it has no controller, to gain that much life (in addition to any other results that damage causes). See rule 119.3.
Lifelink is defined as follows:
702.15b Damage dealt by a source with lifelink causes that source’s controller, or its owner if it has no controller, to gain that much life (in addition to any other results that damage causes). See rule 119.3.
Attacking a Planeswalker deals damage to it.
510.1. First, the active player announces how each attacking creature assigns its combat damage, [...]. A player assigns a creature’s combat damage according to the following rules:
510.1a Each attacking creature and each blocking creature assigns combat damage equal to its power. [...]
510.1b An unblocked creature assigns its combat damage to the player or planeswalker it’s attacking. [...]
510.1c-e [...]
510.2. Second, all combat damage that’s been assigned is dealt simultaneously. [...]
Whatever additional effects the dealing of damage may have (loss of life, gain of poison counters, loss of loyalty counters, gain of marked damage, gain of -1/-1 counters, etc) are irrelevant. You will gain life.
Both the other answers quote a rule which tells you to look at 119.3, but fails to take a look at that rule. Rule 119.3 details what actually happens when damage is dealt. For instance, 119.3c says
Damage dealt to a planeswalker causes that many loyalty counters to be removed from that planeswalker.
And 119.3f says
Damage dealt by a source with lifelink causes that source’s controller to gain that much life, in addition to the damage’s other results.
In other words, a planeswalker losing loyalty counters from receiving damage, and a damage source with lifelink giving its controller life are both equally fundamental to how damage works in MTG. The rules do not say they exclude one another (119.3f even specifies "in addition to the damage’s other results") so both things will happen.
