Error in replacing special chars using awk
 

I have a file with following data :
a$cd
e%gh
I am trying to replace the special chars with space.
When I execute the following command these chars are replaced :
awk '{gsub(/[\041-\177]/,"b",$0);print $0}' temp.txt

Result:
abcd
ebgh

But when I execute
awk '{gsub(/[\041-\175]/,"b",$0);print $0}' temp.txt

Result:
a$cd
e%gh

these chars are not replaced.Could someone tell me if I am doing any mistake.

Thanks in advance

I have to investigate the reason of this strange behaviour more in depth, but in the meanwhile why don't use

Thanks,your solution worked.
If you find the reason for the strange behaviour please let me know.