How can I make Ctrl + Alt act like Alt gr in Ubuntu?

You could add global shortcuts to write special chars. I use this often for my German keyboard layout with Ubuntu (on Windows this work out of the box).

I added the following shortcuts:

Shift+Enter+7 to write the letter {

Shift+Enter+0 to write the letter }

Shift+Enter+8 to write the letter [

Shift+Enter+9 to write the letter ]

To add these shortcuts you will need xbindkeys and xvkbd:

sudo apt-get install xbindkeys xvkbd

Then edit the configuration file:

vim ~/.xbindkeysrc

And add the following lines (edit them as you prefer):

"xvkbd -xsendevent -text '{'"
    m:0xc + c:16
    Control+Alt + 7

"xvkbd -xsendevent -text '['"
    m:0xc + c:17
    Control+Alt + 8

"xvkbd -xsendevent -text ']'"
    m:0xc + c:18
    Control+Alt + 9

"xvkbd -xsendevent -text '}'"
    m:0xc + c:19
    Control+Alt + 0

"xvkbd -xsendevent -text '\[backslash]'"
    m:0xc + c:20
    Control+Alt + ssharp

"xvkbd -xsendevent -text '\[asciitilde]'"
    m:0xc + c:35
    Control+Alt + plus

Then reload xbindkeys:

xbindkeys -f ~/.xbindkeysrc

Thanks to whizz for sharing this (http://forum.ubuntuusers.de/topic/strg-%2B-alt-alt-gr-wie-in-windows/).

After trying and failing to get it to work with a combination of xbindkeys and xvkbd/xte (I could never get certain characters {, [, ], }, \, etc. to behave properly perhaps because I use a Scandinavian keyboard layout), I finally found a simple and practical solution: Autokey. Here are the exact instructions to get it working:

1. Install Autokey through Ubuntu Software (I used the GTK-version, not KDE) or by: “sudo apt-get install autokey-gtk”.
2. Configure one script per key, e.g. for { use this code: keyboard.send_keys("<alt_gr>+7") and set the hotkey to Ctrl+Alt+7 (first set hotkey to 7 then add Ctrl and Alt as modifiers). This at least worked for the most important code-writing keys: { [ ] } .
3. Add autokey to Startup Applications (search for “start” to find it) and then add a program with “autokey” as the command.

This solution will work for anything running in the current X-session (I think) but will not work if you start a new terminal session using Ctrl+Alt+(F2-F6).

Although this still doesn't answer your question, I find this to be an easier solution than the other answers.

To make R-ALT work like AltGr, you can add the following line to your ~/.xinitrc:

setxkbmap -option lv3:ralt_switch

Other ways to shift to the 3rd level (that which is usually achieved using AltGr key) can found, and new ways can be defined in /usr/share/X11/xkb/symbols/level3.