The different OSM editors differ in how you trace from .gpx files. In iD you can drag and drop the .gpx file on the browser window and then trace over it. Uploaded traces are visible in iD on the “GPS trace layer” that you can turn on and off.
Tracing it manually is by far the best way to do it for the very reason you mentioned: it’s a squiggly line. If you just imported the raw track file, it would show every little unwanted deviation where the position accuracy wasn’t very good (like a wavy line where you know the trail was dead straight), a little blob of back and forth where you stopped to take a picture, a gap where you turned off your device briefly, etc. By tracing it manually, you can even things out and eliminate these issues. Yes, it’s more work, but you end up with a better result. I’ve seen a few cases where a raw track file was obviously directly imported, and the result is usually very poor with many of the issues I mentioned above. There are ways to do it in some editors, but I wouldn’t usually recommend it.
In JOSM you can download your area of interest, then import your gpx file, convert your gpx file into a osm data layer and then merge this layer (or copy paste the way) into your default osm data layer (which contains all the already existing osm data).
Wanted to add in my $0.02 here. It may be that the iD editor has changed, but here’s what you do, as of 7/2018. The “iD editor” is where you log into openstreetmap, hit the Edit drop down, and select the first editor option “Edit with in-browser editor, iD Editor.” Then, on the far right of that screen, you’ll see some gray buttons. Hit the layer looking button, which will slide out options from the right. Scroll down to Overlays, and check “Openstreetmap GPS Traces.” Before doing that, you will have needed to upload your GPX to openstreetmaps gpx traces. You’ll see your GPX route overlay, and you can trace it for a new trail.
You generally wouldn’t want to convert and upload a raw GPX file. Raw GPX tracks often have sections where it deviated more than others, jagged or wavy sections where the reality is more straight, and knots where you might have stopped moving. Tracing over top allows you to smooth out and remove these quirks.
I edit all my gpx tracks to follow the real trail in another piece of software that shows the real trails in detail
No one hand drew the trails in my other mapping software, so they are very accurate
You can double check them by looking at them in google earth when the leaves are not on the trees
I only add gpx trails to OSM where there was nothing before
Just a blank forest map
These trails are Pisgah National Forest where I live
Sometimes OSM has the trails listed and sometimes they don’t