Pahoo has the right fix. My first question was going to be are you sure the correct nipple is installed? If it came with the rifle it should be but you don't mention if your rifle is new or used. Or . . if it is new, are you sure that it came with the correct nipple? "Things" do happen at the factory.
Before bending - try giving Track of the Wolf a call and explain your problem. Maybe they have a nipple that is longer that will fit. If they do . . . buy several so you have some spares.
Just as a side note: I'm not familiar with the GPR . . but I have built a number of rifles over the years. Most locks, when the sear is released allowing the hammer to rotate down . . . the hammer hoas "over travel" so that there is no chance of a space between the cup and the top of the nipple.
This may be a wild comment . . . but have you taken the lock off and manually cocked it, released the sear so it dis-engages the tumbler and gently let the hammer lower? Any chance the sear is catching on the last notch on the tumbler instead of by-passing it so that the hammer goes down tight on the nipple?
If I remember correctly, the GPR utilizes a patented breech. I am not saying your problem doesn't exist, but it sure seems like the tail of the lock (the lock itself) would really have to be canted down in the back to keep the hammer from engaging the nipple. If that is the case . . . then your lock mortise had to be cut "really off" and it doesn't seem like the bolster cut-out on your lock plate would fit very well at all.
Another thing to check would be the tumbler itself. Any chance it is a "faulty" tumbler where the geometry is off so that it doesn't allow the hammer to come in contact? Seems like a "long shot" but I have seen such things on locks when replacing internals.
It really is something that Lyman should "address" and make right?
Let us know what you come up with and good luck.