The correct answer is option 2: Gharial.

To better understand why Gharial is not a mammal, let`s dissect the other options:

1. Great Indian Bustard: This is a bird species found in India. Birds are not mammals, as they belong to the class Aves.

2. Gharial: While the Gharial might be an animal, it is not a mammal. Gharials are actually large, fish-eating crocodilians found in parts of India and Nepal. Crocodilians, like all reptiles, belong to the class Reptilia, not mammalia.

3. Platypus: This is an interesting choice, as the platypus is both an animal and a mammal. The platypus is a semi-aquatic mammal native to Australia, and it lays eggs instead of giving birth to live young. Despite this unique characteristic, it still falls under the class Mammalia.

4. Hornbill: This is another bird species. Like the Great Indian Bustard, it belongs to the class Aves. So, it is not a mammal.

In conclusion, out of the given options, the Gharial is the one that is not a mammal