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.
Installing Gramps as an X11 application is achieved by downloading and installing the packages on which Gramps depends using software called fink. The process is a bit longwinded, but most of the features of Gramps work, and drag and drop and mouse clicks work very well. If you choose this route, you may be able to get some help both from the Gramps community and the fink community. Detailed instructions can be found here: Mac OS X fink.
MacPorts is essentially a similar type of program to fink.
See Mac OS X MacPorts on the installation process.
Native Mac OS X
At present (June 2009) there is no downloadable simple installable package for the Mac.
There is however currently an ongoing project to deliver a click-to-install packaged GRAMPS for Mac, but at the moment it has to be built by yourself. 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
An older attempt on PPC mac is described here
Am I ready for X11?
Installing with either fink or MacPorts is simpler that installing as a native application, but the Gramps windows have an X11 appearance. To see whether you can get use dto this appearance, you could download and install OpenOffice which is very easy to do and will show you what an X11 application looks like. Installing OpenOffice will not be wasted, because you will need it to be able to open reports from Gramps.
Another option is to download NeoOffice, which is a port of OpenOffice using Java so that it looks like a native Mac OS X application.
Are there any other options?
(1) Virtual Server
Yet 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) After already having all the dependencies installed
If you have all of the dependent libraries installed already, this should be straightforward. Just download the .tar.gz file, unpack it, and follow the INSTALL instructions.
Is this going to be too complicated for me?
The process is not very straightforward, and there will almost certainly be problems like unresolved dependencies, files not found on the various servers, download of files just stalls indefinitely and questions asked by scripts to which you don't know the answer. Nevertheless, installation is possible and I think gramps is worth the trouble. I suggest you have a look at the instructions for SoulSeek - this is a similar process, but actually SoulSeek is easier because it is fully integrated into the fink process. This might give you some idea of the requirements for perseverance. If you don't think you could install SoulSeek then you certainly won't be able to install Gramps. The converse is not true