We now know the answer to this question, and you won’t be happy with it. The answer to “Is it possible that Mars once had life in the form of tall giant giraffes in its ancient past since Mars had a lower gravity than Earth?” is — no.
We now believe Mars never had enough time to develop multicellular life before the planet died, just as Earth, which is around the same age as Mars, did not have multicellular life on it when Mars died.
However, scientists believe single celled organisms might have developed on Mars in places that had some water over a billion years ago, and the new NASA Perseverance Rover is there to search for signs of extinct single celled life. The cells themselves will not have survived, but some of the chemical signs left behind by organisms might be detected in the samples collected by Perseverance, if we can ever get those samples back to Earth to examine them under a microscope.
We no longer hope for more on Mars.
We don’t hope to find fossils of bones of giraffe-like creatures that developed in the light gravity. We don’t hope to find fossils of trees or vegetation or leaves. We don’t hope to find fossils of fish or clams.
The most we can hope for is signs that single celled microbes may have existed there over a billion years ago.
Such a discovery, however, would completely revolutionize science because it would mean that if life could develop on more than one planet in our own solar system, then life probably exists elsewhere in the universe. As of this moment it is still within the realm of possibility Earth is the only place where life developed in the universe.
But we won’t find evidence of tall creatures ever having lived on Mars.