If you typically load your workouts up to TP from your head unit they won't have the gradients we experience in Zwift as part of the data. Sometimes, seeing the elevation changes in our file can be useful. For example, when looking at an event that you did in Zwift comparing efforts against gradient can be helpful.
When you need to see the gradients you can download the fit file from Zwift and upload it to Training Peaks. You can get the fit file by signing in to your account at zwift.com, click on the workout, then the gear icon, then download fit
file . To get the file to Training Peaks, open Training Peaks and drop the workout fit file from a folder onto the TP calendar. It will load automatically.
The only thing to be aware of is managing those duplicate data files.