how to search for a particular string from a .gz file?
You can use zgrep. Usage is similar to grep.
zgrep "pattern" file.gzFrom the man page's description:
Zgrep invokes grep on compressed or gzipped files. All options specified are passed directly to grep. If no file is specified, then the standard input is decompressed if necessary and fed to grep. Otherwise the given files are uncompressed if necessary and fed to grep.