Old question, but I found this thread looking for an answer. This is what I did to quiet my Gralab 505 timer set-up beeps.
Open the timer case
remove the 4 rubber feet on the bottom of the timer.
Unscrew the 4 screws with a Philips head screwdriver.
Turn the case over and gently pick up the timer case front, and set it aside.
With the timer LEDs on the left, and the power cord on the right, the beep speaker is on the upper right part of the circuit board, right above the upper right switch. It is a small back cylinder with a hole in the middle. I stuck a couple of layers of tape over the speaker hole.
When I reassembled the case, I found it was easier to pick up the circuit board and drop it into the front case. There are 2 plastic pins that line up with holes in the circuit board. Put the side panel with the power cord and outlets in the slot of the front case.
Put the back on, making sure the side panel is in the slot of the back case.
I can still hear the beep, but now it seems more tolerable. If you must have absolute silence, that would involve de-soldering the little speaker from the circuit board.