First there is no direct way to migrate tickets with all information from gnats to redmine. A workaround is to migrate data to bugzilla and from bugzilla to redmine. Install all necessary packages on debian:

bugzilla3
gnats
gnatsuser
gnatsweb (if you wan't to use webfrontend)
redmine
mysql/sqlite (one of these databases)
php5
php5-mysql
apache/nginx (or another webserver)

Installling bugzilla and redmine isn't difficult at all. Create a database for each system and a user for these databases.

For gnatsweb you have to open tcp port 1529 in iptables, because it's the default port gnats uses to communicate.

If using gnatsweb, copy gnatsweb.html, gnatsweb.pl, gnatsweb-site.pl in your /var/www/cgi-bin/ directory.

Then allow gnats to run cron-jobs: echo "gnats" >> /etc/cron.d/cron.allow

Add a new crontab job for gnats user:

crontab -u gnats -e 0,10,20,30,40,50 * * * * /usr/local/libexec/gnats/queue-pr --run

Add the following line to /etc/services: support 1529/tcp # GNATS

Edit gnatsweb.pl and change /usr/bin/perl to /usr/local/bin/perl.

Now run
perl -MCPAN -e 'install CGI'
perl -MCPAN -e 'install MIME::Base64'

Define which hosts can access the gnats-database. Edit /var/db/gnats/databasename/gnats-adm/gnatsd.host_access:

localhost:edit:
x.x.x.x:view: ...

Define which users can access the gnats-database. Edit /var/db/gnats/databasename/gnats-adm/gnatsd.user_access:

gnats:gnatspassword:admin
gnatsuser:gnatsuserpassword:edit ...

After that you've to fix a bug in gnats:

Delete the word function from line 30 of the script /usr/local/bin/install-sid

Bugzilla and redmine in an rudimetarily way work ootb. But for simple datamigration it should work. Now you can test to migrate data first from gnats to bugzilla. Therefor download the bz-migrate plugin from bugzilla and install it to the plugin-folder of your bugzilla-installation. Using the Webfrontend you can import tickets from your gnats-db into your bugzilla-db. That runs mostly acceptable but not without errors. So you have to correct some details manually.

If anything important is migrated you can import the bugzilla-db into redmine. Download the import-scripts from http://www.redmine.org/projects/redmine/wiki/RedmineMigrate. The import from bugzilla into redmine is working without greater errors.

-- SebastianGleicher - 06 Aug 2012