For a transmitting Antenna, can the gain be positive in decibel scale?
I suggest that you read the Wikipedia article about Antenna gain

Antenna gain is defined as the power radiated in a certain direction (so for a directional antenna, you would obviously choose the direction to which most power is radiated) relative to the power if the antenna was omnidirectional. Meaning: it radiates an equal amount of power in all directions.

So a high gain antenna will "focus" all power in one (or more) directions and that is defined as "gain".