Thanks a lot, SGaist
And forgot to mention I figured out the solution to my problem. I had over-specified 'pythonpath in the 'system variable' of the advanced setting which caused the gdb to be unrecognized. when I removed the pythonpath ("something that separately prioritizes language environment") completely then it detected gdb in cmd and Qt. I hope it will help somebody with the same problem. the partial credit goes to a post in StackOverflow which mentioned gdb won't function if the python environment mistakes the actual path even though both normal path and pythonpath are derived from a single installation and single location (which in my case was the python 3.7 only nothing else). The root thing goes to gdb dependency on python scripting extension as was mentioned in Qt guide of debugger installation.
Here goes my windows
error.jpg
error1.jpg