If you have another device on that network that can hand out DHCP (if needed) then you do the following:
Assuming this is a M-Go full stack
1. Create the VLAN on the switch
2. Go to the uplink port on Hardware / GX20 / See All Ports / Select the switch uplink port)/ Settings / Advance Settings / Configure VLAN / Change Allowed VLANs from "all" to the VLANs you want to access the internet.
If you don't have a full stack
1. Create the VLAN on your managed switch
2. Create DHCP pool on your switch or server of choice
3. Go to the uplink on you managed switch and remove the VLAN from that port
From my time with M-Go I did not see a firewall rules section. So to be able to be able to create a rule that says source VLAN X with destination Internet doesn't look to be available at this time. If I am wrong I am interested to be pointed in the right direction of the GUI.