Hello.

I don't think I know how to browse GOG forum, so... I'm just going to post it here...
Admins, if you could find a suitable place for this thread, would you kindly move this post please?

Let's cut to the chase.

First, you will have to set the DOSBox (DBX from here on) renderer to OpenGL. I've tried D3D, but it seems it works sometimes, and most of the times, it won't. Dunno why.

In order to set the DBX renderer, you will have to locate .conf file.

1.1. Stand alone DBX install
For windows Vista and higher, the .conf location is:
HDD(or SDD)\Users\(Account name)\AppData\Local\Dosbox

1.1.1. Open the dosbox-0.74.conf using a text editor.

1.2. GOG game specific DBX install
GOG game specific DBX is installed in:
Game folder\DOSBOX

1.2.1. Open game specific .conf file using a text editor.
GOG DBX .conf naming scheme seems like dosbox(gamename).conf
e.g.) dosboxSWARS.conf, dosboxWC3.conf
Although some games come with more than one .conf file, but you only have to change one .conf file, which contains all basic DOS emulation settings.

2. Find the following setting:
output=surface
This setting is from the default DBX .conf file, depending the game setting this setting might be vary: surface, opengl, overlay, d3d, openglnb.

change output value to:
output=opengl

when changed, save the .conf file.

***Adding Games to Steam client***

3. Go through the "Add Non-Steam games..."
3.1. For the stand alone DBX install, add Dosbox.exe found in the DBX folder.
3.2. If you haven't erased the GOG registered start menu items, select the game you wish to add from the "Add a game" window.
3.3. For the GOG game specific Dosbox.exe, browse to the game folder, then to the dosbox folder within.

4. Right click on the newly added game shortcut, click properties.

5.1. Stand alone DBX should be the simplest of all, nothing to do. It's finished. You can now use the Steam overlay.

5.2. from the game shortcut properties, copy the "Target" inputbox, and paste into the steam's. If's finished. You can now use the Steam overlay.

5.3. GOG game specific DBX should do the following: (Including the quotation marks)
find the game specific .conf file and jot down the name and location of the file (if more than one, write all of them down)
In the "Target:" inputbox:
5.3.1. With one .conf file:
"Game folder\DOSBOX\DosBox.exe" -conf "..dosbox(game name).conf" -noconsole -c exit
5.3.2. With more than one .conf file:
"Game folder\DOSBOX\DosBox.exe" -conf "..dosbox(game name).conf" -conf "..\dosbox(game name_variation.conf" -noconsole -c exit

Remember the second location of .conf file might vary, the two periods (..\) is the root of the very game folder you're trying to tweak. It's finished. You can now use the Steam overlay.

Here are some example from my shortcuts:

"E:\GOG Games\Wing Commander III - Heart of the Tiger\DOSBOX\DOSBox.exe" -conf "..\dosboxWC3.conf" -conf "..\dosboxWC3_single.conf" -noconsole -c exit

"E:\GOG Games\Syndicate Wars\DOSBOX\DOSBox.exe" -conf "..\dosboxSWARS.conf" -conf "..\dosboxSWARS_single.conf" -noconsole -c exit

Anyone else tried this? I just did it with Daggerfall, and the Steam pop-up telling you to press Shift-Tab appeared in game, but pressing Shift-Tab doesn't actually bring up the overlay. just the notifier in the corner when friends log in and out.

Last time I tried this I got the overlay working but it considerably slowed down the emulation. I think I tried it with Theme Hospital (self DOSBoxed, prior to the GOG release).

You can make Steam (or probably any) overlasy to work now. You need to use the branch DOSBOX-daum (has many features the regular build doesn't), and run it in directX mode. The overlay should work then.