There is a weapon proficiency feat; it grants you proficiency with any one weapon of your choice, regardless of what kind of weapon (simple/martial/superior) the weapon is.

If you use a weapon you are not proficient with then you do not receive its proficiency bonus to attack rolls (the character builder is buggy about this and will often show you receiving it, but the rules clearly state that you don't get the bonus if you're not proficient). Nothing reduces this penalty; you could be proficient with every single weapon except one, and you would still not get the bonus for that one weapon.

Looks like a Huge weapon works without penalty

Wield Oversize Weapon says you can treat a weapon as one size category smaller than it is. Thus, you can treat a Huge weapon as if it were Large.

Powerful Build says you can wield weapons one size category larger than yourself without penalty. Since a goliath is a Medium creature, that means a goliath may wield Large weapons without penalty.

Since Wield Oversize Weapon lets you treat the Huge weapon as Large, and Powerful Build lets you wield Large weapons without penalty, you may wield a Huge weapon without penalty.

It does appear that by preventing Wield Oversize Weapon from working with Monkey Grip, the authors were trying to prevent exactly this, which could be an argument for an intent that this shouldn’t work. But by the rules-as-written, that applies only to Monkey Grip, not Powerful Build.

Please note that a size category larger weapon tends to deal only ~1 more damage than its smaller counterpart. There are some exceptions, but it still tends to be small amounts of damage. You can often gain more damage more quickly through other uses of feats. Note also that the size of your weapon does not affect reach (well, larger weapons don’t; too-small weapons do), so you do not get extended reach this way, either.

As for gloves of enlarge weapon, those are a really poor idea. If you meant strongarm bracers, those explicitly do not stack with Powerful Build.