Is it neccesary to have a Ground Instructor Certificate to act as a CFI?

Negative...

61.183 lists the requirements.

Nope. FI > GI, so there's no benefit to doing both of them.

You might think about taking the AGI written test. I believe you need it to get your gold seal.

AGI or IGI to earn Gold Seal. I went the IGI route.

Also, the ground instructor doesn't expire like the CFI does.

Same story with me...got my IGI so I could add the Gold Seal..and yes, your ground instructor certs never expire...for whatever reason.

AND I'll second that having a CFI or II or MEI and no AGI/IGI is good enough to teach your students both in the air and the requisite ground knowledge.

Good question though.

Except you will get out of doing the FOI oral portion on your checkride

Don't count on it - certainly not at the Richmond FSDO.