What do I do about my lab head misattributing my grant proposal?
Is what they're doing ethical? 


Being a chief investigator on a grant does not necessarily imply that you are an author of the proposal.  Proposal a writing can even be outsourced to a third party.

In order to determine if the proposal is ethical, it would be necessary to read it and read the funding agency's rules, but if it's unethical it is probably not for a reason you have stated.


  What can I do if I don't like it?


Probably nothing, except ask the lab head to change their course of action (Is it too late?).  If the university paid you to write the proposal, it very likely belongs to the university.