Logstash gsub: how to replace with backslash?

Hi, I have following stuff in a field:

"field_A" => "c:\\test\\test.txt"

By some conversion the masking backslash became a valid character.
How can I get rid of the double backslashes, so that I have only one (or two when masked)?

I tried:

gsub =>
'field_A', '(\\\\)', '\\'

but that is not accepted by logstash.
If I try to replace to '[\]' then the brackets are also inserted.

Any idea?
Thanks a lot.