Please perform another careful inspection of your module's config.xml.
EXAMPLE (in config.xml , rewrite section)
This is not OK:
This is OK:
Magento uses all whitespaces in new block name, and it cannot find the file with extra whitespaces!
This is pretty hard to debug, but you can confirm it with this call:
$creditmemoItem = Mage::getModel('sales/order_creditmemo_item');
If you have rewritten this class correctly, it should print out Gogi_Cmsubtotals_Model_Creditmemo_Item. If not, it will printout just thi:
Also, you need to either refresh or (better) disable the cache to see the changes online!!!
I hope this tip can save you from some trouble!