I don't know why, but Virb doesn't get power data from gpx or tcx, only from fit files. I try to convert this files to fit, but nothing works.
I think it is understandable, that Garmin supports Garmin products at the first place!
It´s cool enough that you can import gpx files from any device and Virb Edit deals with it. And also the video footage can be from different manufacturers as well.
I think your GPX file contains no power data OR the name of the datafield is different. Open the GPX file in notepad and you can see if the power data is there. GPX is normal txt file.
I have the payed version of strava and the gpx export shows my power:
NOTE You have to have a power meter, the estimated power of strava is fictional and not useful.
<trkpt lat="53.2018270" lon="6.5946670">
Were you using Mac OS VIRB Edit? My GPX file with power data was parsed correctly but the power would not be shown as available under Mac OS. However, VIRB Edit for Windows 10 loaded the same file and made the power data available. I used <gpxpx:PowerInWatts>85</gpxpx:PowerInWatts> where gpxpx is the namespace I used in the file.