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launch_ros.actions.Node inherits from launch.actions.ExecuteProcess, and it passes extra arguments given to __init__ to it. I think you're looking for prefix.

Save this file as gdb_pub_launch.py

from launch import LaunchDescription
from launch import LaunchService

import launch_ros.actions


def generate_launch_description():
    """Run demo nodes via launch."""
    ld = LaunchDescription([
        launch_ros.actions.Node(
            package='demo_nodes_cpp', node_executable='talker', output='screen',
            remappings=[('chatter', 'my_chatter')],
            prefix=['gdb -ex=r --args'],
        )
    ])
    return ld

And run with

ros2 launch gdb_pub_launch.py

launch_ros.actions.Node inherits from launch.actions.ExecuteProcess, and it passes extra arguments given to __init__ to it. I think you're looking for prefix.

Save this file as gdb_pub_launch.py

from launch import LaunchDescription
from launch import LaunchService

import launch_ros.actions


def generate_launch_description():
    """Run demo nodes via launch."""
    ld = LaunchDescription([
        launch_ros.actions.Node(
            package='demo_nodes_cpp', node_executable='talker', output='screen',
            remappings=[('chatter', 'my_chatter')],
            prefix=['gdb -ex=r --args'],
        )
    ])
    return ld

And run with

ros2 launch gdb_pub_launch.py

EDIT

There is new discussion about this answer not being able to interact with GDB because launch doesn't allow a process to access stdin. That discussion is on an issue in the launch_ros repo.

One workaround is to add xterm -e to the start of the prefix to open an new xterm window with the process. In this case, use the prefix xterm -e gdb -ex=r --args.

There's another very similar question and answer in https://answers.ros.org/question/267261/how-can-i-run-ros2-nodes-in-a-debugger-eg-gdb/#q267261

Original answer

launch_ros.actions.Node inherits from launch.actions.ExecuteProcess, and it passes extra arguments given to __init__ to it. I think you're looking for prefix.

Save this file as gdb_pub_launch.py

from launch import LaunchDescription
from launch import LaunchService

import launch_ros.actions


def generate_launch_description():
    """Run demo nodes via launch."""
    ld = LaunchDescription([
        launch_ros.actions.Node(
            package='demo_nodes_cpp', node_executable='talker', output='screen',
            remappings=[('chatter', 'my_chatter')],
            prefix=['gdb -ex=r --args'],
        )
    ])
    return ld

And run with

ros2 launch gdb_pub_launch.py