The Why'sOne Gateway to Rule Them All
You can have as many gateways as you want, you just have to understand the behavior. The "gateway" or more technically "default gateway" or "gateway of last resort", is the last place the device uses to send its traffic. If the device cannot otherwise determine where to send the traffic it will send the traffic to the "gateway". In the routing tables, routes have priorities. So if you have effectively two identical and functional routes (which is what you would have with two default routes/gateways), the tie breaker goes to the priority, and then finally order. In the FPP software, there is no mechanism for you to configure these priorities on the gateway settings, and by default, the gateway assigned to ETH0 is before the gateway assigned to WLAN0. So without the priority setting, if both ETH0 and WLAN0 gateways are configured, the gateway assigned to ETH0 will always win. In the bigger world of networking in general, normally you only have multiple gateways configured for network redundancy and fail over, where you have redundant internet connections, etc.
So the simple answer is, only configure one gateway and configure that gateway on the interface facing the your internet router or show router (if you have one).Subnets, Subnet Masks, and the Magical Third Octet
The Subnet Mask or (sub)NET MASK tells the system what part of the IP address is the subnet address and which part is the host address. Everything eventually gets into binary form, so and IP address 192.168.1.1 is really 11000000.10101000.00000001.00000001 (I added decimals for ease of reading). The subnet mask of 255.255.255.0 converts to 11111111.11111111.11111111.00000000. Starting from left to right, every sequential position that is a 1 means that is the subnet address, the remaining positions which are 0 are the host address.
11000000.10101000.00000001.00000001 (IP Address)
11111111.11111111.11111111.00000000 (Subnet Mask / Netmask / Prefix)
11000000.10101000.00000001 (Subnet Address 192.168.1)
00000001 (Host Address .1)
Octet means "group of eight", which is why we refer to the individual numbers in an IP address as octets because in binary they are groups of eight (highlighted by me adding the decimal after every
You will also find Subnet Mask / Netmask also referred to as Prefix and the notation is /X, X is simply the number sequential 1's left to right of the subnet mask. 255.255.255.0 would be prefix /24 since it has 24 1's.
In IP networks, each separate network (ETH0 and WLAN0 here) must have its only Subnet Address (the first 3 octets here) and on each separate network, each host must have its own unique Host Address (the fourth octet here). The size of the subnet mask as determines how many hosts you can have on the subnet, with 255.255.255.0 you can have 254 hosts (actually its 256, but 0 and 255 are reserved for broadcast use).
You'll probably only ever see 255.255.255.0 subnet masks (/24 prefix), so the short hand to all of this boils down to just make sure "the 3rd octet needs to be different".
And as a bonus, 192.168.X.X (255.255.255.0) is reserved (along with 10.X.X.X, and 172.16.X.X-172.31.X.X for private use, which is way you most commonly see 192.168. subnets in this kind of environment, not to say using the other private ranges are not invalid either).