You can use Audacity which is an audio waveform editing platform, with many plugins and tools for editing audio files. Or you can use a DAW application like Ardour which you can use to edit audio and filter it through plug-ins to get your final product.
There's also abgate which is a noise gate plug-in, that can be used with Ardour. Some others may also get installed when you install Audacity and Ardour, and there are several other plug-in packages available to install as well, which may also include other noise gate options.
You can simply install either of them from Ubuntu Software and then just start using them.