Actually, according to your reference there is both a theory of gravity and a law of gravity.
The law quantitatively documents what happens.
The theory attempts to explain why.
Correct, and that's true whether the theory is proven or not. The point he was making is that theories don't become laws. They're separate concepts. That evolution happens is a fact, and an observable fact. The details of which mutations happened when, where exactly an extinct species lies in terms of being an ancestor to a current species of part of a failed branch closely related to the said ancestor, the role of epigenetics, these things can be revised. As scientists discover more evidence, they refine those details.
There is no law of evolution.
The analogous part you're looking for here would be the law of natural selection. That's a directly observable thing, which is that new species come about as a result of mutation and environmental selection of existing species. Just like the law of gravity, nobody is every going to say gravity doesn't exist, or that evolution through natural selection doesn't exist. The details of how those things happen get refined, but the main thrust of it will never go away any more than Newton's Laws went away with the Theory of Relativity (hey look. Theories superseding laws??? Madness!!)
We can't reliably quantify it.
Buddy, we can reliably quantify so many things about it, it's not even funny. We can build a family tree of species using the same DNA evidence and methods that can be used to build your family tree. We can date fossils at 60 million years old and we can even quantify that uncertainty at about plus or minus a million years. We can quantify the rate of mutations happening in a population. We can examine similarities, and we can tell when certain genes appeared or disappeared. For example, did you know most mammals can make their own vitamin C through absorption of sunlight, as well as vitamin D like we can? Actually, we have that vitamin C creation gene as well. So how come we get scurvy if we don't get vitamin C through our diet? Turns out our vitamin C-making gene is defective, as a result of a mutation. The same defect exists in other primates like chimps. So we can examine the DNA of related species, figure out which ones have the defect and which don't, and you know the mutation first occurred in a species that was the common ancestors to all of those that have the defect, but not all the way back to a common ancestor that encompasses species which do not have a defect, and maintain a working gene.
Which returns us to my thesis: that arguing equivalent confidence in evolution and gravity is as oafish as arguing equivalent confidence in creationism and evolution.
In a way, there is a lot of confidence in creationism. It's provably wrong, we have 100% confidence in that. It can't be refined into something that works, the fundamental idea is incorrect. In the example I gave above regarding figuring out when a mutation occurred, I could have used an example of an additional feature, instead of the removal of a feature (same method. Compare species that have and don't have the feature, feature must have developed after common ancestor to both groups). I chose that one, because it completely disproves not only creationism, but also intelligent design. A lot of creationists like to say, "of course we have so many similarities in DNA. They were all created by the same creator, who re-used the same genes." But given the vitamin C problem, that creator just happened to make a mistake copying that common gene around to his favored species that is supposed to rule the earth. And before you can say, "maybe he didn't want us to be able to have that feature, because he wanted to force us to eat vitamin C containing fruits," you'll have to explain why he made the same mistake with the non-planet-ruling primates.