How to find all files that are larger than 1GB on the Mac HD? It seems that either one of these commands can be used:

sudo find / -size +1G
sudo find / -type f -size +1G


But when it gets to /Volumes, it actually look at all the back up files by Time Machine in the external hard drive. (and another hard drive for data).

I was going to cd to /Volumes/Macintosh HD and start the find there using find . instead of find /, but then I did an ls and there is a Volumes right there again.

Any option to use the Finder as well (the GUI)?

Another thing I found puzzling is that none of the manpage of find on Mac OS Lion or on the web mentioned the +1G... all they mentioned were the format 1G for exact match.
You can use the -x to avoid traversing mounted drives:

-x      Prevent find from descending into directories that have a device number different than that of the file from which the descent
        began.


You command should then be

$ sudo find -x / -type f -size +1G


Regarding the fact about the missing documentation: is not missing is just not easy to find. At the end of the PRIMARIES section of the man page


  All primaries which take a numeric argument allow the number to be preceded by a plus sign ("+") or a minus sign ("-").  A preceding plus sign means "more than n", a preceding minus sign means "less than n" and neither means "exactly n".