The process gain is the ratio by which unwanted signals or interference can be suppressed relative to the desired signal when both share the same frequency channel. For example, if a 1 KHz signal is spread to 100 KHz, the process gain expressed as a numerical ratio would be 100,000/1,000 = 100. Or in decibels, 10log10(100) = 20 dB.
Note that process gain has no effect on wideband thermal noise. On the additive white gaussian noise channel without interference, a spread system requires the same transmitter power as an unspread system, all other things being equal.