Waze can't import a GPX file, a KML file, or any other file. There's no import screen to hunt for. If you went looking for one, that's why you didn't find it.
Why Waze has no import
Waze and Google Maps are both Google products, but they don't share saved places. Waze is built for one live drive at a time, with traffic and road reports. It isn't built to hold a collection of pins. We found no app that can add a list of places to Waze for you, so there isn't a workaround app either. Apple Maps has the same limit, and the Apple Maps page covers it.
Option 1: tap-to-navigate links
Waze publishes a link format that opens the app on a set of coordinates. Build one link per place, save them in a note or a message to yourself, and tap the one you want when you're ready to drive.
The format looks like this:
https://waze.com/ul?ll=37.7749%2C-122.4194&navigate=yes
Swap in your own latitude and longitude, and keep the comma as %2C. On a phone with Waze installed, the link opens the app and starts navigation. Without the app, it opens Waze in the browser.
To get the coordinates, paste your Google Maps list link into the converter above and export it as CSV. Every place gets a row with a latitude column and a longitude column. The CSV page has the details.
In a spreadsheet where latitude is in column B and longitude is in column C, this formula builds the link for row 2. Fill it down for the rest.
="https://waze.com/ul?ll="&B2&"%2C"&C2&"&navigate=yes"
Option 2: search the coordinates and save a Favorite
Paste a coordinate pair, like 37.7749, -122.4194, into Waze's search bar. Waze drops a pin there. Save it to Favorites and it stays in your account. Read the numbers off the converted GPX in a text editor or a GPX viewer, or take them from the CSV.
Which option to use
Option 1 is faster if you have a spreadsheet handy and want a short list of destinations to tap through. Option 2 needs nothing but Waze. Both are manual, and the free tier converts up to 25 places per list.
Past ten places or so, Waze is the wrong home for the list. A GPS or hiking app handles a long list of pins properly. The Gaia GPS and Garmin pages walk through two of them, and the GPX page covers the file itself.