Please close and reopen your terminal/command prompt/Studio/IDE after you add GAMS System directory to your environment variable PATH.
After you do this, you should be open command prompt and use gdxxrw and it should identify. Similarly, the command gams should be identified as well. If it isn’t you are setting incorrect path.
The path is usually like “C:\GAMS\35”. (gams.exe, gdxxrw.exe exists in this path)
Hope this helps.
- Atharv