crontab error with (No MTA installed) but I use >/dev/null 2>&1

Of course, the error is that you don't have a mailer (sendmail,postfix, etc) implemented and active.

That being said your other problem is that the >/dev/null 2>&1 ONLY only applies/associates to the LAST command in this case gzip. Thus there must be some type of output going to STDERR for your mysqldump.

The correct way to do what I think you want is:

 * * * * *  (command | command ) >/dev/null 2>&1

You can put:

MAILTO=""

at the beginning of your crontab file and that will prevent it from trying to send email.