At what point can I start playing against the bosses in Naxxramas successfully

The only way to know a secret is, is to reveal it or through process of elimination figure out what it is not. There is only 1 card currently to negate secrets, Flare.

In order to use process of elimination you need to know what secrets are out there. Each class has a fixed cost for their secrets, so that will give you no information regarding what it is. The other thing to take into account is the game format and the regularity with which each secret could be expected.

If you are playing Constructed in "Play" mode and your opponent uses 1-2 legendaries on you, it is safe to expect they have access to the rare and epic traps; but if they are playing primarily the basic cards it is more likely they may not have the rarer secrets.

If you are playing Arena, there are certain secrets you see more often than others. For example, most players rate Explosive Trap fairly highly as a Hunter and it is a common.

That said, there is also some basic logic you should use when testing a trap. If you are attacking a hunter, be intentional about which creature you attack with (knowing that the trap could be a freezing trap), this can sometimes be used to your benefit. If the trap does not trigger when you attack, then it is not Misdirection, Freezing, Explosive, or Snake... leaving only Snipe. So then be intentional about which creature you play next. Also, you would want to attack before you play a creature in almost every scenario because of the high probability of Explosive Trap or Misdirection.

3 classes have secrets; Hunter, Mage, & Paladin.

Hunter:

Snake Trap (epic) - trigger: attacking minion, effect: summon 3 1/1 snakes (beasts).
Explosive Trap (common) - trigger: attacking player, effect: deal 2 damage to all enemy Characters (includes you).
Freezing Trap (common) - trigger: any minion attacking, effect: return minion to hand & it costs +2 to play.
Snipe (common) - trigger: "Play" a minion (not "Summon", so for example it does not trigger from paladin/shaman abilities or imp master), effect: deal 4 damage to that minion.
Misdirection (rare) - trigger: When any character attacks a hero, instead he attacks another random character.

Mage:

Ice Block (epic) - trigger: mage takes fatal damage, effect: mage doesn't die this turn, stays at 1 life.
Spellbender (epic) - trigger: target any minion with a spell, effect: randomly select a new target.
Counterspell (rare) - trigger: cast any spell, effect: that spell is countered.
Vaporize (rare) - trigger: attacking player with minion, effect: destroy that minion.
Ice Barrier (common)- trigger: attacking player, effect: mage gains 8 armor.
Mirror Entity (common)- trigger: "Play" a minion, effect: summon a copy (battlecry abilities do not trigger for mage's copy).
Duplicate (common - Naxxramas) - trigger: friendly minion dies, effect: Place 2 copies of destroyed minion in hand

Paladin:

Eye for an Eye (common) - trigger: paladin takes damage, effect: after paladin takes damage the same amount is done to his opponent.
Noble Sacrifice (common) - trigger: attacking anything, effect: paladin summons a 2/1 and your attack is redirected to that.
Redemption (common) - trigger: any of paladin's minions die, effect: return to life with 1 health.
Repentance (common) - trigger: you play a creature, effect: it's health is reduced to 1.
Avenge (common - Naxxramas) - trigger: friendly minion dies, effect: give a random friendly minion +3/+2

There are 2 great ways you can use Baron Geddon.

OPTION 1 (available to any golden legendary)
Since you got a Golden B.G. you could disenchant him for 1,600 dust which would allow you to craft any other non-golden Legendary of your choice (if you are not a huge fan of him).

I believe most people consider him to be one of the better legendaries, but above him yet is still (definately) Ysera and Ragnaros the Firelord. Leeroy Jenkins can be a great surprise game-closer as well.

OPTION 2
Second, you build him into your decks (some of them).

I do not have one yet, but I have had him work well in 2 arena decks where I had the good fortune to draft him. He is best used in a late-game style control deck.

I believe the best way to use him is in a Mage deck with +Spellpower (2 kobold geomancers, 2 ogre mages, & 2 azure drakes, +Malygos?) and a lot of spells. Arcane Blast, Blizzard, Flamestrike serve to wipe the board, especially with the benefit of spellpower.

In that kind of deck, Baron Geddon functions as an additional board wipe that also puts out a ton of pressure on your opponent's health.

He could be good in a druid deck because you could get him out earlier, also druids can be built with a lot of spells, similar to the mage and he could add additional control. He probably is a little worse in a Warlock deck than in most others because he competes with your ability for your limitted resource of health. He is probably not great for a shaman or paladin deck because he limits the functionality of your ability.

He will not work as well in an aggressive deck that uses a lot of small 2 & 3 cost minions. Try him out in a few different situations and do some unrated games in "Play" mode and report back here and let everyone know what you found out.

TL;DR: My opinion
Mage: +++
Druid: +++
Priest: ++
Warrior: ++
Rogue: ++
Paladin: -
Hunter: -
Shaman: --
Warlock: ---