What kind of fish is long and has a round fin with whiskers like catfish greyish in color buries itself under the gravel tank all time?

Based on your description, the fish you're describing is most likely a Corydora catfish.

Here's why:

* Long and Greyish: Corydoras are elongated fish with a generally greyish color, often with darker markings.

* Round Fin: Corydoras have a distinct, rounded dorsal fin (the fin on their back).

* Whiskers: They have prominent barbels (whiskers) around their mouths, which they use to sense food.

* Buries in Gravel: Corydoras are bottom dwellers and love to sift through gravel, often burying themselves partially for protection or to search for food.

There are many species of Corydoras, each with slight variations in appearance and behavior. However, the common features you described strongly suggest that you have a Corydora catfish in your tank.

To be certain, you could try searching for pictures of different Corydora species online to see if you can identify the exact type you have.