Yes you can use Drano Max Gel in standing water.
The Drano Max Gel is thick enough and formulated to be able to be poured into the standing water to help clear the clog.
I've had good luck pouring the Drano Max Gel directly into the standing water to clear the clog.
If possible get most of the water out of the sink, tub etc so that the Drano Max gel works better.
I usually use a water pump to pump water out of the bathtub when it is full and gets clogged.
I just pump the water into the toilet.
You can also use a bucket to scoop the water out.
But Drano Max Gel can work in standing water to clear clogs.