I was searching for a single line to downscale 1080p video to 720p / H264. Here we are :
gst-launch filesrc location=in.avi ! decodebin ! videoscale ! \ video/x-raw-yuv, width=1280, height=720 ! x264enc pass=quant ! \ avimux ! filesink location=out.avi
Very good quality, work like a charm 🙂