While you can use the OpenCPN Route and Mark manager and just "Import GPX", this will import essentionally unknown information into your personal OpenCPN data.  This is the easiest way, but not necessary the best idea.

The OpenCPN Route & Mark Manager has a tab called Layers.  Layers are a way to add groups of information from GPX files (tracks, routes, and waypoints) in a way that you can easily show or hide the whole group, and remove the whole group, with one click.

Instead of importing someone else's GPX files into your OpenCPN data, you can either temporarily view someone else's file as a Temporary Layer, or add it as a Persistant Layer.

Temporary Layers
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Temporary Layers will automatically be removed from OpenCPN (but not delete the file from your original location) when you shut down OpenCPN.  This is useful for checking out what someone else has handed you in a GPX file.  

Persistant Layers
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If you check out a GPX file using the Temporary Layers function, and find it is useful information that you want to keep in OpenCPN layers (until you remove it), then import it again as a Persistant Layer.  Then it will stay until you explicity remove it using the Delete button.

To do this, click on "Create Persistant Layer" and then navigate to the GPX file you wish to include in OpenCPN.  Once created (imported), it will show up in the Layers list.  You can hide or show each layer individually using the eyeball icon on the left side of the list of layers.

More Advanced Control over OpenCPN Layers
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For more advanced control over layers you can manually manipulate layers files into folders.  For example, you might have several people's information in separate files for Fiji.  You can create a "Fiji" folder, and put the separate GPX files into the Fiji folder, then you can show and hide the entire Fiji "layer" (ie not individual files as separate layers anymore).

To do this, you have to shut down OpenCPN, find the OpenCPN folder in Program Data (Windows) or Android/data/org.opencpn.org, find the "layers" folder, and manipulate the individual files into sub-folders.  The contents of the layers folder is only read in at startup of OpenCPN.