Well, on the ethereal plane that the ghost resides (is that correct plane?), the Hill Giant's Str does matter. On that plane, they are completely solid. A ghost touch weapon, essentually exists equally in both the ethereal and material plane, so the ghost's strength does matter. If it were a ghost touch glove, you could shake hands with the ghost, for example, or arm wrestle it. It is just a physical to you as it is the ghost on your respective planes of existance.
Actually, the ghost of the gnome might actually hit "harder" since your ghost is going to be hitting using his dexterity. The reason that this makes sense is because, well, strength is the amount of force that your body can bring to bear on things. When your body is less substantial than air, you are going to have a hard time being effective with it.
We ran into this problem recently with a batch of villains. We have a group of ghosts, one of whom is a storm giant. This storm giant's strength (his primary offensive factor) is now nil and he has sort of rolled into a mastermind role. Most of his CR has gone to waste, but I think it makes for an interesting villain. The little gnome sorceress is the bruiser and the enormous storm giant is the thinker.
This is a case where I think turning to the 3.5 material could be most useful. The book Libra Mortis had a lot of information on ghosts and how to handle them in the different planes. For example you could gate a ghost back to the prime material plane making him completely solid... he would still be a ghost, therefore undead and having most his abilities, however he would also be subject to the normal effects of the prime material plane. Any ways I would suggest if you are going to mess with ghosts a lot that the above book would be a good place to do some research.
Think of it like this. you have a ghostly 30 lbs rock. Normally, if someone "dropped" the rock on you, it would go right through you, (and technically not have any weight, but for the sake of argument, it weights 30 lbs).
Now, if it is a ghost touch ghostly 30lbs rock, it does weigh 30 lbs, and you will feel it all when it is dropped on you. You can also see it perfectly, because it is physically present on your plane (as well as the ethereal plane) equally.
note that there was a big change from the 3.5 ghost to the pathfinder ghost... in 3.5 a ghost resided on the ethereal plane, where he was corporeal, and had a strength score.... but manifested on the material plane as an incorporeal version of itself... in pathfinder ghosts reside on the material plane and are simply incorporeal (just like a wraith or a specter).
normally when a creature with out an ability is forced to do something using the ability in question it treats it as a 10 (no bonus or penalty), and this is what I would probably do in the case of a ghost (or wraith or specter) using a ghost touch weapon.