I have a BKOOL cycling trainer which records power. When I import the GPX file into Garmin Connect for my training ride, all data is present apart from the power data, here is a section from the GPX file so you can see the data is present.
<trkpt lat="54.522633673623204" lon="-3.221423048526049">
<ele>132.2</ele>
<time>2015-05-20T11:20:42Z</time>
<extensions>
<gpxtpx:TrackPointExtension>
<gpxtpx:atemp>0</gpxtpx:atemp>
<gpxtpx:cad>70</gpxtpx:cad>
</gpxtpx:TrackPointExtension>
<gpxpx:PowerExtension>
<gpxpx:PowerInWatts>129</gpxpx:PowerInWatts>
</gpxpx:PowerExtension>
</extensions>
</trkpt>