First, download and install the free Malwarebytes for Mac, run it to see if it detects and cleans any malware, then you can remove it via its Help menu.
In the Terminal, run the following command to list everything not belonging to the operating system (com.apple) that was started by launchd:
launchctl list | egrep -iv 'com.apple' | more
and for more info:
man launchctl
You can control the behavior of a notification from the System Preferences > Notifications panel by selecting the specific notification name and changing its settings, or you can select it and press the delete key to remove that notification.
The gcaptain.com site is a maritime news platform, that on first visit, provides a drop-down panel offering to let it send notifications to you. Did you agree to that, or install any software from their site?