I just want to make sure I'm understanding you correctly first!
Initially, there's a physical interface (eth5), connected to a switchport, the switchport is configured as a trunk port with VLAN1 untagged/native, and VLAN200 tagged, and an IP on the gateway physical interface.
Then the gateway interface is reconfigured so that it is using VLAN200, as eth5.200.
Is everything else that you're trying to communicate with on VLAN1 or VLAN200?
Is VLAN1 using a different subnet? (Or is your intent to just lift-and-shift VLAN1 to VLAN200?)
What device is providing inter-VLAN routing between VLAN1 and VLAN200?
You've said that you can't connect to anything else in that subnet apart from the gateway, what are you using to connect to the gateway and how? What other devices exist on the network, and how are they connected? (i.e. by which switch/switchport, and how is that port configured, etc).
Lots of questions I'm afraid!
I hesitate to suggest this, but my initial suspicion is that you've moved the 192.168.200.254/24 interface from VLAN1 to VLAN200, and everything else in 192.168.200.0/24 is still in VLAN1, hence the firewall can't see it at all...