Mac OS X:Build from source
There are three different ways of installing GRAMPS on Mac OS X. The linked pages provide step by step instructions.
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 MacPorts on the installation process.
Fink X11 version
Similar to MacPorts, see Mac OS X fink.
A native version of GRAMPS is under development. This version will not require X11 and so will look more 'Mac like' and integrate better with the OS X desktop.
The new project is described at Mac gtk-osx port. Patches to GRAMPS to fix some of the issues are being included for version 3.1.3/3.2.x. However it is still a work in progress and not recommended for daily use. There is no binary release so you will have to compile it yourself from source.
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 X11 MacPorts option is very straight forward. The other options may be more complicated.