The OEMV Installation and Operation Manual and RF Equipment Selection and Installation documents are referring to two different types of measurements. The acceptable RF input range for the OEMV1 card (as an example) is listed as -80 to -105 dBm, which is an absolute value (referenced to 1milliwatt). This means that the RF input on the V1 card expects an input of -80 to -105 dBm from whatever antenna is used.
The +15 to +30 dB value quoted is a NET GAIN value, and is relative. What this means is that we expect -95 to -120 dBm of signal to hit the antenna itself, however, that is very hard to measure. The dB GAIN value that we quote is easy to measure and is what you need in order to properly determine what antennas and cables will work for you.
In terms of the OEMV card, you will need to provide it with a -80 to -105 dBm signal no matter what antenna or cable you use, that is what the card must get as an input RF value. In terms of the antenna and cable package, you will need to determine how much gain you need to provide to the signal in order for it to be in the -80 to -105 dBm range. For a typical GPS satellite signal, +15dB of gain is acceptable.
If you are working in an environment that has poor signal strength or quality, you may want to aim for more gain (up to +20dB will be good enough for nearly all applications). Once you know how much gain your system needs, subtract that value off of the amount of gain the antenna will provide, this will tell you the maximum allowable loss in dB that your cable can have.
For example, this is using a NovAtel 702 antenna, which provides +27dB of gain, so: 27 dB 15 dB = 12 dB gain. That means that using a 702 antenna and a +15dB net gain, you can have a maximum of 12dB loss in the cable assembly. This will give the receiver the desired RF signal input range.