Hi,
For comparing h264 and h265, please use same YUV source and compare PSNR.
For example, you may download the source:
https://media.xiph.org/video/derf/y4m/park_joy_1080p50.y4m
Run
$ gst-launch-1.0 filesrc location= park_joy_1080p50.y4m ! y4mdec ! nvvidconv ! nvv4l2h264enc bitrate=14000000 control-rate=1 vbv-size=450000 ! h264parse ! qtmux ! filesink location= park_joy.mov
$ gst-launch-1.0 filesrc location= park_joy_1080p50.y4m ! y4mdec ! nvvidconv ! nvv4l2h265enc bitrate=14000000 control-rate=1 vbv-size=450000 ! h265parse ! qtmux ! filesink location= park_joy_265.mov
And install/run ffmpeg to check PSNR
$ gst-launch-1.0 filesrc location= park_joy_1080p50.y4m ! y4mdec ! filesink location=park_joy_50.yuv
$ sudo apt install ffmpeg
$ ffmpeg -i park_joy.mov -f rawvideo -video_size 1920x1080 -framerate 50 -i park_joy_50.yuv -lavfi "ssim;[0:v][1:v]psnr" -f null - ffmpeg version 3.4.6-0ubuntu0.18.04.1 Copyright (c) 2000-2019 the FFmpeg developers
(...skip)
[Parsed_ssim_0 @ 0x55c102f010] SSIM Y:0.713943 (5.435473) U:0.732030 (5.719138) V:0.788833 (6.753741) All:0.729439 (5.677350)
[Parsed_psnr_1 @ 0x55c102eda0] PSNR y:24.882475 u:30.801891 v:34.448052 average:26.262739 min:22.971376 max:30.380294