Use first or last free IP for the default gateway?
I am of the "put the gateway at the top of the subnet" religion. It makes absolutely no difference, though.
For kicks, I've put the gateway in a /23 on the ".0" or ".255" address (i.e. 192.168.0.0/23, spanning 192.168.0.0 to 192.168.1.254, with 192.168.0.255 and 192.168.1.0 both being valid host IPs)... That's some good fun, that. I enjoy watching the heads of people who don't understand IP subnetting asplode when I tell them-- "The default gateway is 192.168.0.255..."
I would use 192.168.100.1, then asign IP starting at 192.168.100.10 (I keep some free IP if I need to add some network device like load balancer, etc.)
Why ? Imagine you need to shrink the network to a /25, if you have used 192.168.100.254 you must change the gateway all host.
Ok you will tell me, I can use 192.168.100.254 and start affecting IP from the top instead of the bottom then shrink to 192.168.100.128/25 instead of 192.168.100.0/24. That's right.
Now imagine you need to extend the network to 192.168.100.0/23. If a gateway set as 192.168.100.1, the gateway is still the first IP of the network when 192.168.100.254 would be in middle.
That why I prefer to take the first IP most of the time, it's also easier for others person, they just know that the gateway if the first IP in the subnet and most of the time changing the subnet will not affect this statement.
From what I've seen, it's almost entirely personal/organizational preference. As long as you are consistent, neither way is wrong. If you really wanted to, you could set up the router to have both IPs (but this would be silly.)
Less important than "is the gateway high or low?" is "did you document what you did so so in the future people will know how to find things?"
There is no hard and fast rule about this, but on nearly all of the networks I've designed and/or worked on, the lowest IP in the subnet is used for the router/default gateway. In your case, this would be 192.168.100.1.
As for your "why?" question - it's really just a convention that the networking community has settled on. If you want to put the default gateway somewhere else, feel free, but really you may be asking for confusion at some point in the future.