He was not the only one who knew of the Chamber...
It is true that the Chamber was not easy to find, and that Riddle, an extraordinary individual, had difficulty finding it:
It had taken me five whole years to find out everything I could about
the Chamber of Secrets and discover the secret entrance...as though
Hagrid had the brains, or the power!
—Harry Potter and the Chamber of Secrets
However, for many centuries the Heirs of Slytherin knew of the Chamber:
The existence of the Chamber was known to Slytherin’s descendants and
those with whom they chose to share the information. Thus the rumour
stayed alive through the centuries.
Nor was he the only one who opened it
As nobody else could hear the creature sliding beneath floorboards or,
latterly, through the plumbing....
This implies that previous generations of Gaunts had let the basilisk out, but had been unwilling to take the final step of committing murder with it.
Further, Rowling states it explicitly:
There is clear evidence that the Chamber was opened more than once
between the death of Slytherin and the entrance of Tom Riddle in the
twentieth century. When first created, the Chamber was accessed
through a concealed trapdoor and a series of magical tunnels. However,
when Hogwarts’ plumbing became more elaborate in the eighteenth
century (this was a rare instance of wizards copying Muggles, because
hitherto they simply relieved themselves wherever they stood, and
vanished the evidence), the entrance to the Chamber was threatened,
being located on the site of a proposed bathroom. The presence in
school at the time of a student called Corvinus Gaunt – direct
descendant of Slytherin, and antecedent of Tom Riddle – explains how
the simple trapdoor was secretly protected, so that those who knew how
could still access the entrance to the Chamber even after newfangled
plumbing had been placed on top of it.
In other words, given that the plumbing in Hogwarts is superior to what Muggles had at the time it was built, and given that wizards are technologically behind Muggles, at some point the Chamber entrance must have been modified, which (according to WoG) implies that Heirs wanted continued access to the Chamber.
But he was the only one who used the basilisk
Whispers that a monster lived in the depths of the castle were also prevalent for centuries. Again, this is because those who could hear and speak to it were not always as discreet as they might have been: the Gaunt family could not resist boasting of their knowledge. As nobody else could hear the creature sliding beneath floorboards or,
latterly, through the plumbing, they did not have many believers, and
none, until Riddle dared unleash the monster on the castle.
If no one believed the stories told by the Gaunts until Riddle unleashed the basilisk, this is a definite implication that this was the first time the basilisk was used as Slytherin had intended—to kill Muggle-borns. The word "dared" also implied that Riddle was the first.