Trevor Godfrey was told he would not have been sacked even if he had not already retired over his involvement in the case of 19-year-old Grice and her violent co-worker Michael Lane.
The teenager was murdered by Lane at her home in Brighton on 25 August 2016.
It later emerged that she had reported her former boyfriend to police five times in six months but was fined for wasting officers' time after it emerged she misled them in initially failing to disclose the pair had previously been in a relationship.
The case was closed before her pleas for help were properly investigated.
A misconduct panel sitting in Lewes found Mr Godfrey failed to adequately investigate allegations of harassment and stalking, and that he failed to treat Grice as a victim - behaviour which amounted to a breach of police rules.
Recording the decision, panel chairwoman Victoria Goodfellow said: "While clearly serious matters, they are not enough to mean gross misconduct. We do not attribute any blame on Shana."
There was little emotion shown from either Mr Godfrey or Grice's family.
The hearing was told how Mr Godfrey was a police constable tasked with investigating an allegation of common assault on 25 March 2016 after Grice claimed Lane chased and made physical contact with her while taking her phone from her ear.
During an interview, she made a number of other allegations against Lane, including that he had sent her unsolicited flowers and had made numerous attempts to contact her.
However, after being arrested, Lane told Mr Godfrey the pair were actually in a relationship - and provided details of mobile phone messages between the couple which backed his story.
Grice later confessed the pair were having an affair behind the back of her then-boyfriend Ashley Cooke.
Mr Godfrey, during an 84-second phone call to the teenager, informed her she would be fined for wasting police time over the harassment allegations - a decision ratified by police bosses.
Mr Godfrey stood by the decision when giving evidence to the misconduct panel.
He said: "She lied to police three times. It was only right I advised her she cannot keep lying in police statements and getting people arrested for it."
James Berry, counsel presenting the case against Mr Godfrey, also accused the former police officer of applying an inaccurate stereotype to Grice that she could not be at risk from a man she was in a relationship with.
Mr Godfrey replied: "No. It may be the case (applying stereotypes) for other people, I don't have those views."
He added: "There was no history of violence between them, there was no evidence of violence, or risk, at that time."
Mr Godfrey said there was no sign of Grice being harassed, something she previously accused Lane of, before admitting to police that him being outside her house late at night was because she had arranged the meeting behind the back of then-boyfriend Mr Cooke.
Mr Godfrey said: "She would be signing her texts (to Lane) with five kisses. This is not harassment. It was a smokescreen to disguise her affair."
Mr Godfrey also said there was therefore "no reason" to supply Grice with safety advice regarding her relationship with Lane.
He said: "She was in an active relationship with him for six months. I can honestly say, hand on heart, there was nothing there to suggest she was in any form of danger whatsoever."
Summing up the case against Mr Godfrey, Mr Berry said the former officer "did not show the expected level of diligence" when dealing with Grice's allegation against Lane.
He said there was a "serious failing" in Mr Godfrey not filling out a risk assessment form following this incident.
He added: "Mr Godfrey applied the stereotype (of a person in a relationship not being at risk of harm from the other) to Shana's statement because it affected how he viewed her, how he viewed her allegations, whether he thought he needed to carry out a risk assessment, and whether to give her safety advice.
"Mr Godfrey simply lost his impartiality and Lane became the victim and Shana became the wrong-doer. Everything Shana said carried no weight any more - even the things that were not in doubt. Mr Godfrey's handling of this case did not represent the impartiality expected of a police officer."
Representing Mr Godfrey, Mark Aldred said: "It is easy to lapse into: 'The officer should have done, ought to have done, could have done it differently...' This is about gross misconduct - so there must be evidence or a positive duty to do something in a particular way. Decision-makers don't make a decision in a vacuum."
Lane, 27, was jailed for life with a minimum term of 25 years after being convicted of her murder.