In games with blinds, such as Texas Hold'em or Omaha, it is the player seated just to the left of 3️⃣ the big blind. The under the gun player must act first pre-flop in a game with blinds. After the flop, 3️⃣ the under the gun player is then the third to act, after the small blind and big blind.
UTG is also 3️⃣ used as a shorthand for the other early positions, with UTG+1 being the next player to the left of the 3️⃣ under the gun position, UTG+2 the second player to the left, and UTG+3 the third player to the left.
Disadvantages of 3️⃣ the Under the Gun Position
The term under the gun implies that you are under pressure, and that is true for 3️⃣ this position. Everyone is awaiting your play before the flop and you don't know what the others are planning to 3️⃣ do.
Pre-flop, all of the players at the table will have the option to call, raise, or fold after the UTG 3️⃣ position. When you are in this position, you have no information about the strength of the hands of the other 3️⃣ players. You don't know whether any of the other players will be calling, raising, or folding, and how many will 3️⃣ still be in the hand after the flop.