As per the chapter Harishchandra of Brahma Purana
"How can I obtain a son?" asked Harishchandra.
"Go to the banks of the Goutami Ganga," advised the sages.
So this Ganga was not Bhagirathi Ganga but Goutami Ganga.
If Ganga was already there then why Bhagirath brought Ganga again?
As per Bala Kanda of Valmiki Ramayana, Lord Shri Rama ruled for 11,000 years and his earlier generations even more (Anshuman - 32,000 years, Dilipa - 30,000 years). So from 33rd to 44th generation there is huge gap in term of years.
It might be possible that Ganga was on Earth earlier but due to some reason left the Earth. And later Bhagiratha brought Ganga to Earth again.
This might be same as
Goddess Lakshmi was emerged during Samundra Manthan but before that she was born as daughter of Khyati.
Similarly, few stories are related to Ganga:
As per SRISHTI KHAND - Padma Purana (page 96), Ganga originated from Lord Vishnu's feet.
As per the chapter Vishnu and Manu of Matsya Purana, Ganga was there before the the beginning of Vaivasvata Manavantar i.e. (even when Ikshvaku was not born). But as deluge (प्रलय) happened after that so many people brought Ganga on Earth again.
I. As per Last Chapter of Skanda Purana,
Lord Brahma instructed Ganga by saying- ' Ganga! Go to the earth, where you would be known as Gomti. Sage Vashishtha will lead you to your destination. Just follow him like a daughter follows her father.'
II. As per the chapter The Origin of Trayambakeshwar - PART FOUR KOTI RUDRA SAMHITA of Shiva Purana, Sage Gautam requested Lord Shiva to liberate him from the sins of killing a cow. He also requested lord Shiva to manifest the stream of river Ganga that place. This Ganga became famous as Gautami Ganga.
III. Daughter of Himvana (sister of Parvati)
IV. Bhagiratha brought Ganga on Earth, this Ganag is known as Bhagirathi Ganga.
V. As per Sambhava Parva - Mahabharata, Lord Bhrahma cursed both Mahabhisha and Ganga to take birth in Mrityu loka (Ganga in Mahabharata as wife of Shantanu)