Does a creature with both deathtouch and lifelink gain life from damage dealt to a creature with indestructible?

Lifelink causes you to gain life equal to the damage dealt by the creature, as a static ability that applies whenever that creature deals any damage (CR 702.15b). It does not matter whether the damage results in the target being destroyed — only that damage was actually dealt.

Indestructible means a permanent cannot be destroyed by damage or by effects that say 'destroy' (CR 702.12b). Crucially, damage is still dealt to an indestructible creature — it just doesn't die from lethal damage. The damage event itself occurs normally, which is all lifelink cares about.

Deathtouch means any amount of damage that creature deals to another creature is considered lethal (CR 702.2b). However, against an indestructible creature, deathtouch's 'lethal damage' designation still applies but the creature simply cannot be destroyed as a result (CR 702.12b). The damage is still dealt, so lifelink still functions.

Example: Your 2/2 with deathtouch and lifelink attacks, and your opponent blocks with an indestructible 5/5. Your 2/2 deals 2 damage to the 5/5. The 5/5 survives (it's indestructible), but you still gain 2 life from lifelink because 2 damage was dealt.

In summary: lifelink looks only for the dealing of damage, not its consequences. Indestructible stops destruction, not the damage event itself, so lifelink will always trigger when damage is dealt to any permanent, indestructible or not.