There is a Primer that is included with the Firefly download.  I'm pretty sure there are other primers available for the other plugins as well.  You may start by clicking on each group's page (lower right hand corner of the grasshopper home page).

Hi Ivo

for Geco just join our group and take a look on our example files

http://www.grasshopper3d.com/group/geco/forum/topics/example-files-...

Kangaroo and ghowl have examples attached with there plugin

For Geometry Gym visit his blog http://geometrygym.blogspot.com/

to download examples

best

to]

for galapagos, the principles of the solver are here :

http://www.grasshopper3d.com/profiles/blogs/evolutionary-principles