If I ambush Rengar against an attacker and become a defender, if I win combat with Rengar, do I gain a point?

Yes, you can gain a point in this scenario, provided you have not already scored that specific battlefield earlier in the same turn.

According to the rules for combat and conquering (Rule 461.5, Rule 464.1) and the precedents established in RiftJudge FAQ #11014, here is how the process works:

The Sequence

1. Opponent's Move: Your opponent moves a unit to an unoccupied battlefield. This initiates a non-combat showdown and makes the battlefield Contested (Rule 445). Your opponent is designated the Attacker.
2. Your Response: You play Rengar, Trophy Hunter as a [Reaction] to the contested battlefield. Because he allows you to be played to a battlefield with enemy units, this is a legal play. Upon entering the battlefield, you are designated the Defender (Rule 442.1.a.2).
3. Combat: Once the chain clears and both players pass focus, the showdown transitions into a combat showdown.
4. Victory: If you win the combat (you are the only player with units remaining at the battlefield during the Combat Cleanup step), you establish control of the battlefield.
5. Score: Establishing control of a battlefield that you have not yet scored this turn constitutes a Conquer action (Rule 464.1). As long as you have not already scored that specific battlefield earlier in the turn, you will gain 1 point.

This interpretation is based on the rules for establishing control, combat resolution (Rule 461.5), and specific RiftJudge FAQs regarding conquering on an opponent's turn (e.g., FAQ #11014, FAQ #3657).