FW fly is a method of generating images "on the fly" using GD, and allows for command line access (via, say, Tcl's exec) so it can be used by just about any language with shell access and I/O. This is, as far as I know, the only way of using GD from Tcl.
http://martin.gleeson.com/fly/installation.html
FW's knowledge is incomplete; refer to the GD page for other options.
As of 2012, there is also TclGD from flightaware, which provides GD as an extension.