What version of grbl firmware are you using? There is a J-Tech LaserMode grbl 9g hex that can be downloaded on this forums main page. It executes S0-S255 commands in real time and outputs PWM on pin D11 & ground. If your TTL driver can handle a PWM frequency of 1khz. it should work to vary the laser's output power to laser engrave shades.
With the J-Tech grbl these settings need to change.
$0=30 (step pulse, usec)
$1=255 (step idle delay, msec)
$10=18 (status report mask:00010010)
$11=0.005 (junction deviation, mm)
$12=0.002 (arc tolerance, mm)
$14=1 (auto start, bool)
$110=2540.000 (x max rate, mm/min)
$111=2540.000 (y max rate, mm/min)
$120=8000.000 (x accel, mm/sec^2) <----------Play with the X&Y accel settings for your machine.
$121=8000.000 (y accel, mm/sec^2) Our Shapeoko 2 is set at 12000 for the X&Y.
This grbl firmware will not run arc commands or a G0/G00 rapid move. It's setup to just run raster gcode files for laser engraving photos/logos.