While I really don't know how much it costs to manufacture these things, we can still do some analysis.
Cost difference estimation done with the help of Newegg.
Processor
The Gateway has an i3 according to Gateway's website, so I'm not sure about where the i5 is coming up. Either way, it's probably worth about $50-$100 more than a Core 2 Duo. As has already been pointed out in this thread, once Apple updates the MBP line, this advantage will go away.
Graphics
Another discrepancy - Gateway says this laptop has no dedicated graphics, but you claim it has a 128MB dedicated graphics card. The MBP has a 256MB dedicated nVidia card. If your laptop does have a dedicated card, I'd still put that at $50 MBP advantage. If it doesn't, that's a whopping $300 or so in the MBP's favor.
Operating System
Let's just say, for kicks, that Windows 7 and Mac OS X are roughly equal in value.
Memory
4 GB 1066 MHz DDR3 on the Gateway, 4 GB 1066 MHz DDR3 on the MBP. A perfect tie!
Hard Drive
320 GB 5400 RPM on the Gateway, 500 GB 5400 RPM on the MBP, for about a $40 difference.
Display
15.6" LED 1366 x 768 on the Gateway, 15.4" LED 1440 x 900 on the MBP. Unfortunately, I really don't know how to tell the price difference for an extra 246,912 pixels, but I'd guess it's on the order of $50 or so.
AC Adapter
No Magsafe on the Gateway, Magsafe on the MBP. Maybe $30?
Chassis and Weight
Plastic (?) 5.84 lbs on the Gateway, Aluminum 5.5 lbs on the MBP. Sturdier design and 0.34 lbs lighter, let's put the price on that around $30.
Battery
??? on the Gateway, 7 hours on the MBP. I'm guessing about $100 difference here.
Ports
4 USB, HDMI and VGA on the Gateway, FW800, 2 USB, and DisplayPort on the MBP. Maybe $20 in Gateway's favor, if for nothing other than having more ports and the potential cost of a DisplayPort adapter.
Card Readers
Multicard Reader on the Gateway, SD card reader on the MBP. Maybe $20 in favor of Gateway.
Optical Drive
Blu-Ray/DVD on the Gateway, Superdrive in the MBP. $50 in favor of the Gateway for the Blu-Ray.
Other than that, I'm not seeing many significant differences. That's a total of $110-$410 in favor of the MBP, which is almost 8 months old (about a generation in terms of technology).
So why doesn't the Macbook Pro cost $600-$1000?
Well, another thing to take into consideration is Apple's environmental efforts. Mercury-free LED display, arsenic-free glass, ENERGY STAR, PVC-free internal cables, etc. Even if you don't care about the environment, there's a cost associated with environmental-friendliness.
Furthermore, as others have pointed out, there's Mac OS X versus Windows 7. I'm sure Apple thinks OS X is worth more than Windows 7 due to a plethora of factors (stability, ease of use, etc). And then there's iLife.
Of course, when Apple brings out the next generation of MBPs, that'll probably add $500 in updates/upgrades, and they'll be selling that for the same price as the current MBP.
So, yes, a MBP is expensive. They're probably making a good profit. A 30% profit margin puts the MBP around $1700, which, if you look at everything I've laid out, is about right.