Yes, you can redirect Guttersnipe's damage to a planeswalker, due to the rule you cited. (That's an actual rule, not a ruling or someone's interpretation.) Guttersnipe is dealing noncombat damage to a player, you are that player's opponent, you control the Guttersnipe, and you may therefore redirect the damage.
But saying "planeswalkers are classed as 'players'" is only going to get you into trouble. They are not players. They're, well, planeswalkers. There are a few things you can do that make them feel like players: you can redirect damage like this, and you can choose to attack them or your opponents with your creatures. But they aren't players. If a card actually says "player", it means a player, not a planeswalker.
- A burn spell like Lightning Strike ("...deals 3 damage to target creature or player") can only target a player. You may end up redirecting the damage, effectively using it on their planeswalker, but you still have to target your opponent. If they have a Witchbane Orb ("You have hexproof"), you can't target them, so you can't cast the spell in the first place.
- Gray Merchant of Asphodel ("...each opponent loses X life...") doesn't do anything to your opponents' planeswalkers - you can't redirect loss of life. Similarly, you can't somehow use life gain on a planeswalker.
- Dreadbore ("Destroy target creature or planeswalker.") doesn't do anything to players.
Most of the time things are pretty obvious anyway - for example you're not too likely to think a planeswalker can draw cards and try to use Opportunity on it. But you can avoid the chance of confusion by just remembering what you can do (attack and redirect damage) rather than thinking of planeswalkers as players.