Mac OS X:Build from source
A fully packaged binary application for Gramps on Mac is available for both Intel and PPC and its use is described at Mac OS X:Application package.
This page describes a number of different ways of building and installing GRAMPS from source on Mac OS X. The linked pages provide step by step instructions.
The fully packaged binary application uses GTK-OSX and is the newest port. It installs as an app bundle which you can double-click in Finder to launch. The build process is almost fully automated, just follow the step-by-step instructions. Your mac came with everything you need except Apple Xcode, which is free from the App Store for Lion users and is on your distribution DVD for earlier versions of OS X.
MacPorts X11 version
MacPorts is a packaging system for automatically downloading, compiling and installing Unix software on Mac OS X. It makes the process simple and almost entirely automatic.
See Mac OS X:Build from source:MacPorts on the installation process.
Fink X11 version
Similar to MacPorts, see Mac OS X:Build from source:fink.
An older attempt on PPC mac is described here
Are there any other options?
(1) Virtual Server
Another option is to install a virtual server on your Mac (like VMware Fusion), create a virtual machine running Linux, and install GRAMPS in there. A Gnome based distro like Ubuntu would be a good choice for the Linux system.
(2) Compile GRAMPS from source without a package manager
If you have all of the dependent libraries installed already, this should be possible. Just download the .tar.gz file, unpack it, and follow the INSTALL instructions.
If you have Bootcamp installed, you can use the Windows version of GRAMPS.
Is this going to be too complicated for me?
The GTK-OSX option can build Gramps from source on the Desktop without typing a single character! The X11 MacPorts option is very straight forward. The other options may be more complicated.