To add multiple gateways, use option 3 once but with a list of IP addresses:
Like I said, option 2 is subnet mask and is automatically set to the mask of the VLAN for which you're configuring the DHCP options. That's why Meraki doesn't allow you to customize that.
Edit: Also, that second Option 3 shown in your screenshot is actually suboption 3 of Option 43 (all of the MSUCClient ones actually). For Skype 4 Business you need to configure those. More info here:
http://blog.schertz.name/2012/05/understanding-dhcp-option-43/