Tobin, 21, had denied being to blame for her fall at his home on Lower West End in Port Talbot on April 24, 2015.
But he was caught by police after detectives put a secret recording device in his home to hear him confess the killing to his father.
She said: "Through her death, she has gone on to save the lives of six others."
Mrs Gillard said: "My last memory of my beautiful daughter was her in hospital fighting for her life.
"She didn’t stand a chance with James Tobin. I am no longer capable of smiling, let alone laughing - all my happiness is gone."
Her family said her decision to donate her organs "sums up Kellie's kind and giving nature".
They said: "We are proud that Kellie received the Order of St John award as a result of her organs being donated and saving the lives of others. "
Kellie's family described her a "a much loved daughter, sister and auntie."
In a statement, they said: "Kellie was a amazing, beautiful person, full of life and energy. She lit up our lives and we were blessed to have had her as part of our family."
They said they are devastated and will never get over her death and said they are "grateful" Tobin has finally been brought to justice and accepted responsibility for her death.
Tobin was later arrested on suspicion of murder but pleaded guilty to manslaughter. He was jailed for 16 and a half years on Friday.
In a victim impact statement to Swansea Crown Court , Kellie's mother Tracey Gillard said she had agreed to become an organ donor before her death.
Acting Detective Inspector Peter Collins, from South Wales Police's major crime investigation team, said: "As always, our thoughts are with the family and friends of Kellie Gillard who I hope have been given some comfort by the conviction and sentencing of James Tobin.
"The tragic death of Kellie brought shock and sadness to the community of Port Talbot – and I would like to thank the local community for their support during the investigation.
“We welcome today’s sentencing which brings an end to the criminal investigation and I again pass on my condolences to the Gillard family who have acted with great dignity and humility throughout this horrific ordeal."He added: “Tackling domestic abuse is a priority for South Wales Police. I urge anyone who is a victim to seek help and support from the excellent partner agencies that provide assistance and expert advice.