Gratitum Slow at max rank is suppose to be a 30%, decaying over 3.5 seconds, as stated in the ability description
In the Practice Tool, it seems to slow for 30% for less than or equal to 1 second, and instantly decays to 10% for the remainder of the duration.
I tested it vs Ryze, who had 340 MS, it went down to 238, but after less than a second it jumped back up to 306 for the remainder of the duration.
238 / 340 = 0.7 (30% slow)
306 / 340 = 0.9 (10% slow)
Also the Gravitum R states that the slow is 99%, however it reduced the MS from 340 -> 112.
112 / 340 = 0.329411 (about 66% slow)
I believe the rate of decay of the slow is bugged or incorrect, and the R slow is not correctly slowing for 99%, or the description of the ability should be changed to 66%.