The difference is that when you "lose" gold it still exists. If your hard drive crashes and you don't have your bitcoin wallet backed up your bitcoins cease to exist.
Actually no, they still exist, you just can't access them without the private key.
In theory, given enough computational power [unlikely] one could manage to find/re-generate the same private key and would thus have access to those bitcoins.
Of course, if anyone ever had that much computation power available to them it would probably make the rest of bitcoin unviable.
So from a practical standpoint the bitcoin are lost, but they still technically exist.
Pedantic I know. Perhaps one could compare it more to that gold being 'lost' by being converted into some other compound [ie. auric chloride]. The gold is unusable as a currency in that form and thus is 'lost' from that standpoint. But, given the technical know how and resources, you could presumably recover the gold from the compound but it would be difficult and expensive.