Why is it not suitable to use a soldering gun to solder modem electronics component?
Solder guns as a rule have wider tips than soldering
irons/soldering pens.You might not be able to reach just the spot
that you want to with a solder gun given the size of the
components, and how tightly packed they are on a modern circuit
board.
Many modern components, such as integrated circuits, will be
damaged by the high heat levels produced by soldering guns.