Temporarily compressing MySQL binlogs
Yes, it is perfectly safe to do so as long as you don´t gzip the file which is currently written (the last file). 

Also in case of a replication setup ensure that the files you are gzipping are already fetched by the slave. You can verify this by checking the output of SHOW SLAVE STATUSon the slave, look here for Master_Log_File, this will give you the logfile of the master which is currently be fetched by the slaves io-thread. Every preceding file is absolutely safe to gzip.