GEDCOM format doesn't appear to bear any useful resemblance to OmniGraffle's format, and while it might not be too difficult to write a tool that took a GEDCOM file and made some sort of document in OmniGraffle, going the other way seems more difficult. One big obstacle would be interpreting the meaning of your diagram to determine all of the relationships and so on. Remember, all OmniGraffle really understands about your document is that it is a bunch of graphics and text, and unless you were very fastidious about how you constructed it, building and using a conversion tool might take longer than just entering the data again in the new program. Without knowing how you represented your data, or how much of it there is, this sort of hand-waving is probably all you will get. It is sort of like the difference between taking a string of characters from a file and printing it on the page, vs. taking an image of a page and trying to turn it back into a string of characters, without knowing if the original was a single column, multiple columns, etc.