You have 10th and B.Sc marksheets having your mother's name as Gita Devi. But your 12th has her name as Gita.
Generally during document verfication, these minor issues and spelling mistakes should not create problems but it is safer and wiser to have the 12th marksheet changed so that it is in line with your other education certificates.
How about your other ID proofs (birth certificate, caste certificate if any and PAN card)?
Have ID proof copies of your mother that has her full name. Get the xerox copies attested and then contact your 12th school or college adminstration office.
You need to draft a letter requesting the service you want; this along with the other documents and the neccessary fees will be forwarded to the State 12th board by the principal.
It takes on an average 30 days for this service, the respective board will send the new certificate/marksheet to your 12th school or college.
If you 12th standard is from CBSE. then you need to follow the guidelines mentioned here (http://www.cbse.nic.in/faq/revised_dob_name_corr_rules_2015.pdf)
This is the form for Name correction services
(http://www.cbse.nic.in/faq/MM%202016%20Duplicate%20Form.pdf). You also need to have an affdavit of your mother's correct name as you want. This affidavit will be give by a Notary. The rest of the procedure is more or less the same as above.