If the guy above me’s tip didn’t help, here’s a more thorough tutorial:
1: Start Gmod.
2: Click Options
3: Click Keyboard.
4: Click Advanced
5: Click “Enable Developer Console”
6: If pushing ~ works for you, skip to step 11. If pushing ~ does nothing, continue with the following steps:
Close Gmod again and do this:
1: Go to the games tab in steam
2: Right click Gmod
3: Click Properties
4: Click “Set Launch Options”
5: Type “-console” without the " ".
6: Click OK
7: Click OK
8: Start Gmod.
9: The console will now be up.
10: Type “bind F12 toggleconsole” without the " ".
(You can change the F12 key for any key you want, but I suggest F12.
11: Type “bind B bullettime” without the " "
(You can change the B key for any key you want, but I suggest B.
12: Done. You will now be able to open/close your console with F12 and bulletime will be toggled with B.
If you want it so that you must HOLD DOWN B for it to be activated, simply write “bind B +bulletime” instead. Without the " ".
I hope this helps you.