It catches things, but not by the gross (3)
I believe the answer is:
net
This might be a double definition.
'it catches things but not' is the first definition.
The definition suggests a singular noun which matches the answer.
'the gross' is the second definition.
(thesaurus)
'by' is the link.
Can you help me to learn more?
(Other definitions for net that I've seen before include "land" , "Trap; open-weave fabric" , "Catch; fine fabric" , "catch fish" , "After taxes" .)