Well I'm currently completing a PhD on basically this topic of the gospel and the law / gospel distinction in reformed thought (with a focus on John Owen). So you can read all about it soon ...
Frame's conclusions don't have much to do with the FV IMHO. Rather he is rightly noting that there is a difference to how the Lutherans and Reformed construe the law / gospel distinction.
To be sure certain reformed theologians (e.g. Richard Greenham) had a Lutheran law / gospel distinction. But on the whole there was a basic difference, particularly with the rise of federal theology.
Lutheranism:
Law = commands (imperative)
Gospel = promise (indicative)
Reformed
Law = covenant of works ("do then and live" - commands and then promise)
Gospel = covenant of grace ("live and then do this" - promise and then commands)
One crux is whether the gospel contains commands. For example, does the gospel call us to repentance? For Lutherans no, for the Reformed yes. But it is a repentance that arises from being justified (not to be justified).
The gospel commands repentance out of reconciliation with Christ. But my
actual repentance is not the gospel.
However, Frame perhaps is incorrect in this: even if the Reformed tradition defines the law / gospel distinction differently to Lutheranism, let us be sure of one thing: there is a
sharp distinction between the law and the gospel in
both traditions. (I perhaps wonder if this is what the WSC guys are attempting to communicate, but at times they sound a little Lutheran--particularly when they use the language of "imperative vs indicative").
God bless brother.