I am not a gentoo user. However assuming you get to login after booting then as root
cat /var/log/dsmeg > /var/log/dmesg.1
should save the current boot process's dmesg file prior to a crash. /var/[debug,messages,syslog][.x] files as these are commonly rotated/archived should capture events after booting and prior to a crash.
However if you never get to login then thats another kettle of fish. I started to advise modifying your startup script at the point where the file system is remounted read/write to use the above code. But what is the point if you can't login to see it. Suspect gentoo has a way to run a 'live' or abbreviated install where you would not proceed with the install rather just mount under /mnt your partitions and then you should be able to read the last dmesg file.
Just a thought. Hope it helps