I uploaded "Slow Gear Attack Pre" to the Rig Exchange ( before I uploaded it, it was called "Slow Gear Attack PreDelay")
It uses the Reverb's Predelay control to create a similar delayed attack effect to the Boss Slow Gear pedal.
I have it set short, so it will track fast playing, but you can adjust it to be faster or slower.
I have the Reverb Time set to "0", so that it sounds like the pedal, without a reverb. If you want to hear some ambience, add a little Time.
Here is a short example: