Much of the time, and probably in this case, just plain less will do the right thing. This uses the so-called less preprocessor. So, do
less file.sql.gz
This won't actually write the file in plain text to the filesytem; it is for your viewing pleasure only, but much of the time that is enough. I personally find the less preprocesor quite useful. It saves me from remembering a bunch of commands. On Debian, at least, the less preprocessor is /bin/lesspipe. This is a shell script.
The relevant part of this script for just a gzipped text file is:
# Note that this is out of alpha order so that we don't catch
# the gzipped tar files.
*.gz|*.z|*.dz)
gzip -dc "$1" ;;
This is with less version 444-4 on Debian wheezy.