in these matters, SOP is to search poser forum, user bagginsbill.  docs re: tonemapping in poser 8 are not in abundance, but try these two:

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2778431&page=23#message_3572246

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2778431&page=23#message_3572198

maybe bill or others have done the curves for tone-mapping with variable gains to emulate GC curve, and have found such empirical curve-fitting to be unsatisfactory.  it may be necessary to purchase poserpro if GC is desired and tonemapping/gain curves can't be adjusted sufficiently.