sub and gsub function?

I have this command:

$ find $PWD -name "*.jpg" | awk '{system( "echo "  $(sub(/\//, "_")) ) }'
_home/mol/Pulpit/test/1.jpg

Now the same thing, but using gsub:

$ find $PWD -name "*.jpg" | awk '{system( "echo "  $(gsub(/\//, "_")) ) }'

mol@mol:~

I want to get the result:

_home_mol_Pulpit_test_1.jpg

Thank you for your help.

EDIT:

I put 'echo' to test the command:

$ find $PWD -name "*.jpg" | awk '{gsub("/", "_")} {system( "echo " mv $0 " " $0) }'
_home_mol_Pulpit_test_1.jpg _home_pic_Pulpit_test_1.jpg

mol@mol:~

I want to get the result:

$ find $PWD -name "*.jpg" | awk '{gsub("/", "_")} {system( "echo " mv $0 " " $0) }'
/home/pic/Pulpit/test/1.jpg  _home_pic_Pulpit_test_1.jpg