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My understanding of GPL2 license make impossible that a proprietary software like "Viscosity" could possibly include a GPL2 code like OpenVPN.
But going to their page, they clearly state that they are based on OpenVPN. Following the legal documentation: https://www.sparklabs.com/company/legal/ it is not clear how they solve it.
How OpenVPN can be legally included in Viscosity?
There is a huge difference between "based on the OpenVPN code" and "configures the OpenVPN client". As you say, something which is based on (derived from) the OpenVPN code must be licensed under the GPL. On the other hand, something which simply configures the client (probably by writing config files) is a "mere aggregation" with the OpenVPN code and can be under any license.
It's likely Viscosity configures the OpenVPN client, rather than being based on the OpenVPN code itself.
Reading what they offer it looks to me like this :They ship a network security solution. That consists of several pieces of software. One of them is Openvpn. Others are config tools for the VPN and network settings etc. As such it falls under the clause that they ship aggregates of different programmes which may well have all their own license.
(I cannot verify without actually buying their stuff though)
