well you can goto the "material_template_default.txt" and find the "CHITIN" template and add in: [MATERIAL_REACTION_PRODUCT:TAN_CHITIN:<insert material group>:<inster material subgroup>] -- What this does is class all chitin as TAN_CHITIN and attaches an extra material to it, say a standard attachement is
[MATERIAL_REACTION_PRODUCT:TAN_CHITIN:LOCAL_CREATURE_MAT:SPIDER_STEEL] --- looks for a material in the creature tagged as SPIDER_STEEL, now since you altered the entire CHITIN template butchering other creatures that yield chitin will flip a b**ch when put through the reaction.
So if you want only GCS to have this goto the creature and after the [BODY_DETAIL_PLAN:xxxxx_MATERIALS] toss in:
[REMOVE_MATERIAL:CHITIN]
then
[USE_MATERIAL_TEMPLATE:CHITIN:CHITIN_TEMPLATE]
[MATERIAL_REACTION_PRODUCT:TAN_CHITIN:LOCAL_CREATURE_MAT:SPIDER_STEEL]
then under it
[USE_MATERIAL_TEMPLATE:SPIDER_STEEL:METAL_TEMPLATE]
*copy all values from INORGANIC:STEEL but change the name*
now that the creature has an internal "steel" material we can go to out reaction
[REAGENT:gcs chitin:1:NONE:NONE:NONE:NONE][USE_BODY_COMPONENT][UNROTTEN][HAS_MATERIAL_REACTION_PRODUCT:TAN_CHITIN] --- targets the chitin
[PRODUCT:100:1:SKIN_TANNED:NONE:GET_MATERIAL_FROM_REAGENT:A:TAN_CHITIN] --- this makes a leather item from the internal "SPIDER_STEEL" material
EDIT:
the rest is up to you, hope this helps.