If you have been using the DOS version of Grafx2 (or still using it through DOSbox), the following information will tell you how Grafx2 adapts to the new platforms. Even if you've been using the Win32 port by Eclipse-game, some of this information may interest you.
Of course, the main change is that you don't need to run MS-DOS anymore. GrafX2 is now running on a very wide range of operating systems and CPUs.
Long file names are supported. You can see up to 28 characters from the file name in the Save and Load windows, and you can scroll to the left and right for even more. The character encoding used is Windows-1252, but there is some limited support for UTF-8, too.
The "select drive" button allows you to reach the drives (Windows), volumes (Amiga), or mount points (Linux). This replaces the DOS-specific drive icon handling. The list is refreshed each time you click the button, so you can add new drives (USB drive, for example) while the program is running.
On all platforms, the files are sorted in a case-sensitive way, but the "quicksearch" feature is case-insensitive. There is also a new set of bookmark buttons to quickly reach your favorite directories.
The mouse cursor should use exactly the settings of your OS. Grafx2 no longer needs its own mouse settings. The program allows you to bind shortcuts to your third mouse button and mouse wheel, in combination with shift / ctrl / alt if you want. 4th mouse button (and more) is still not supported.
You can type filenames normally, the right characters will appear (within the limits of Latin-1 character set). Pressing control C in text input boxes no longer crashes the program. The configuration of keyboard shortcuts is done directly from the help screen, no need for a separate configuration tool anymore.
The configuration files from the DOS version are still recognized, and GrafX2 will try to migrate as much of your settings as possible.
The program's default mode (and 'safety resolution', the one you can call with Shift+Enter) is now a scalable window, since this is guaranteed to work on all platforms and screens. You can resize or maximize the window using your normal window manager, dragging the window edge etc. The other modes are fullscreen, using whatever technology provides fullscreen modes on the OS (DirectX on Windows, X11? on linux) The program has a preset list of (low-resolution) modes, and checks with SDL if each of them is supported. It also queries SDL for more resolutions, and all modes auto-detected by SDL are added to the list. The resolution screen only shows video modes that SDL claims it can display. If your graphics cards reports a mode that outmatches your monitor's capacities, you can tag it "Unsupported" by clicking the left-column button until it displays a black block. This way, GrafX2 will not use this mode on your system.
About color depth: Grafx2 is still a 256-color program, and will use 256-color mode if they are available. If your hardware or driver only support truecolor/hicolor modes, it will use them instead, SDL does an excellent job of converting on-the-fly 8b->24b.
Since it is uncommon for recent hardware (and drivers) to support low resolution video modes like in the DOS days, GrafX2 now offers an additionnal software scaling. This allows to double or triple the size of the pixels in software, and also allows wide (2x1) or tall (1x2) pixels, as required by some old graphics (C64, Amstrad CPC, Amiga, ...)
The color precision is now 24-bit, instead of 18 in the DOS version (which was limited by the VGA hardware). This can result in some changes to the saved files, for example the maximal color value saved by GrafX2 was 252, not 255, in GIF files.
The original GrafX2 is a single-tasking DOS program, it uses all available CPU cycles. This version reduces the cpu usage to what's actually needed, so it will not overheat your machine.
A lot more file formats are now available, including more modern ones such as PNG. The old custom PKM format is not recommended, instead GrafX2 will now use GIF with some specific extensions to store its own data.
All missing features from the beta versions under DOS are now implemented, and a lot more. The list is quite long (Animations, layers, text, gradients, ...), so maybe you should re-read the complete user manual.
Fortunately, the help system was also improved. You can press the F1 key at any place to get help. This work in all windows, and also while hovering a button in the menu with the mouse. This is also the place where you can set your keyboard shortcuts.