MAX3232 CAE outputs -6V / +6V instead of -12V / +12V
I need to connect this module to a third party system that only accept RS232 with -12V / +12V signals levels.


You will need to supply details of that device, and a copy of the documentation which leads to the conclusion that it requires those signal levels. That requirement is not part of RS-232.


  When viewing MAX3232CAE TX pin trace with an oscilloscope, i can see it outputs -6V / +6V signal.


Good.


  Wasn't MAX3232 supposed to output -12/ +12V signal


No, the MAX3232 datasheet shows that it outputs a minimum of +/-5 V (with 3k load) which meets the RS-232 specification (see TIA/EIA-232-F paragraph 2.1.6).


  This GPS module also exists in version with TTL output rather than RS232, if i go with TTL outputs (0-5V) how could i get a real -12V / +12V RS232 output since MAX3232 looks not to do so ?


+/-12 V is not "real" RS-232. Even a very old RS-232 driver, like the MC1488, is not guaranteed to output +/- 12 V. Although it would be possible to achieve those signal levels, it might require a design using discrete components or boosted power rail voltages for an old RS-232 driver. 

In the distant past, +/-12 V was a common maximum RS-232 voltage level seen in real life. Perhaps the documentation you have for that "module" has been mistranslated or otherwise misinterpreted that this voltage is a requirement...