I'm on day 6 of Gabriel Knight Sins of the father. I know I was supposed to go to Mosley's office before officer Frick came back, but he came back before I got a chance to go in. Now he won't leave. I went to do other stuff that I was supposed to do. What do I have to do to make him go to lunch again?

SPOILERS
If you wait about frick will take a nap at his post and you can get passed him then.

You'll have to go into his office and talk with him again to tricker him to leave...
then do as above poster said.. wait a bit outside and watch as Frick dozes off - then go through the swing door and wait again...

If you ever replay the game, the solution to get past the police while he is on to lunch is to lower down cpu cycles from dosboxGK config file. In my own game reducing cycles during play didn't have any effect, but when I edited the config file to following: core=simple and cycles=6000 (It probably wouldn't have been that low, but I wanted to be sure that it works) I had plenty of times to go past that scene before the cop came back.

I also have a patch for it http://sierrahelp.com/Patches-Updates/Patches-Updates-Games/GabrielKnightSeriesUpdates.html#GK1_NRS

What destination do I need to install thispatch to? And which version...the CD or floppy?

Strange. I never waited for Frick to fall asleep. Everytime I played the game after waiting for a short moment, the beignet seller arrives and Frick goes outside to buy some.

That's how it is supposed to work, but on faster computers he comes back before you have time to do anything. Therefore lowering DOSBox cycles or otherwise slowing down your computer is necessary, if you want to get past it.
This issue isn't limited to just old Sierra games, but other old games have similar issues, for example in Blade Runner it's necessary to slow down the computer, because otherwise it's impossible to escape exploding bomb.

Hey; maybe it was a similar but separate thread that I saw a solution to this problem before, but I do not see that particular post in this thread.
Isn't there some sort of Ctrl+? or Shift+? key fix for this, that you can press in-game to slow down DosBOX's processing or game speed or something? I tried it and that seemed to work, and that is IMO a very easy solution. Does anybody remember or know what I am referring to?

It's a standard DOSBox feature. Ctrl+F11 decreases cycles (slow down) and Ctrl+F12 increases cycles (speed up).

Here's the full list of shortcuts:

ALT-ENTER Switch to full screen and back.
ALT-PAUSE Pause emulation.
CTRL-F1 Start the keymapper.
CTRL-F4 Change between mounted disk-images. Update directory cache for all drives!
CTRL-ALT-F5 Start/Stop creating a movie of the screen. (avi video capturing)
CTRL-F5 Save a screenshot. (png)
CTRL-F6 Start/Stop recording sound output to a wave file.
CTRL-ALT-F7 Start/Stop recording of OPL commands.
CTRL-ALT-F8 Start/Stop the recording of raw MIDI commands.
CTRL-F7 Decrease frameskip.
CTRL-F8 Increase frameskip.
CTRL-F9 Kill DOSBox.
CTRL-F10 Capture/Release the mouse.
CTRL-F11 Slow down emulation (Decrease DOSBox Cycles).
CTRL-F12 Speed up emulation (Increase DOSBox Cycles).
ALT-F12 Unlock speed (turbo button).

Dont waste your time on getting past this part. Obviously its bugged and changing this in the config will allow you to get past it.

Doesn't take much practice really. All you have to do is speed up. Set the speed setting as high as it will go and when the cop nods off, click on Mosely's office door.