Ok.. might want to tweak the math a bit more then?
According the the setgravity page, "is a fraction of the server-wide gravity"
If I enter a gravity number 1 to 2000, and you multiply it by 600, won't that number always be more than 1?
Find a way to use between 1 and 100, make the user type it in understanding that, for example, ulx gravity <player> 50 is 50% of the server gravity. Use math to change the actual number from a real to percentage... have that decimal percentage applied to set gravity.
According to the setgravity command, you shouldn't have to multiply anything.
Make it a decimal 0 to 1, and you've got it set.