tooltip-gud-modes

tooltip-gud-modes is a variable defined in `gud.el'.
Its value is

(gud-mode c-mode c++-mode fortran-mode python-mode)

• This variable is an alias for `gud-tooltip-modes'.
• This variable is obsolete since 22.1;
  use `gud-tooltip-modes' instead.

Documentation:
List of modes for which to enable GUD tooltips.

You can customize this variable.