I made a plugin for Illustrator CS6 that creates roblox GUIs out of path art. Took me forever and it’s my first (+last) time working in a low-level language so it’s lacking a few features and may bug.
https://drive.google.com/folderview?id=0B2nH4ELRPxhTLXdRUThLeWgwUW8&usp=sharing
Included source so smarter people can upgrade or port to Mac and other versions of Illustrator! If you’re successful, please share in this thread. 
If you don’t know how to add plugins to Illustrator, put the .aip file and the .ilk file (idk what it does but it’s better to be safe and use it anyways) in Illustrator’s Plugins folder:
Windows: C:\Program Files\Adobe\Adobe Illustrator CS6\Plug-ins
Mac OS: /Applications/Adobe Illustrator CS6/Plug-ins/
To use it, just go to File>Export and select RBXM File.
Basically, the plugin creates Frames when it finds rectangular paths with an RGB fill. When the path isn’t a rectangle or has a different color type (i.e. gradient), it creates an ImageLabel and a corresponding PNG for you to upload and link. The PNGs are created in the same folder as the RBXM file, so I suggest exporting to an empty folder. All text frames are also exported as ImageLabels, so only use them for static text (titles, button labels, etc). I couldn’t create TextLabels since the same fonts in Roblox look a lot different than they do in Illustrator.
Layers and objects are converted into transparent Frames that hold all of their contents.
Drawbacks:
[ul]
[li]Doesn’t create buttons 
. You’ll have to turn ImageLabels into ImageButtons in Studio.[/li]
[li]Doesn’t use BorderSizePixel. Rectangular paths will strokes will be exported as ImageLabels.[/li]
[li]After uploading images (especially text), they may look a little “fuzzy.”[/li]
[li]Doesn’t create Frames for CMYK-color-filled paths. You’ll have to set your document’s color mode to RGB.[/li]
[/ul]
Something I threw together:
The “a film by” and “MrHISTORY” elements are both ImageLabels under the same Frame. In Illustrator, they were two text frames under the same Layer. Basically shows how it works. 
