I have found a solution, but it means modifying 2 bytes in mirc.exe. This disables "deflate" CRC checking and allows you to fed $decompress() with gzip compressed data (after a bcopy).
Validity of the decompressed data can then be vefiried with $crc() but normal deflate format will loose any crc verification.
If you're connection with http, another solution can be to simply use:
Accept-Encoding: deflate
instead of:
Accept-Encoding: gzip,deflate
And if the site(s) you connect to supports deflate, it's enough. But most sites only support gzip
