tl,dr: I don't see an economic argument for GN marriage, or marriage in general what-so-ever.
All spending on marriage, are nothing more than consumption goods. There is no reason to believe that marriage-related spending have a higher Keynesian multiplier than other consumption categories.
As long as this is the case, as @denesp argues, "money spent on marriage" is money that would instead be spend on different consumption goods. Legalizing marriages would be a boost to the economy, and useful in crises and slumps - but again, by Keynesian logic, the same would be true for burying money in the ground. There is no reason it has to be marriage specifically.
However, the real world is no frictionless environment.
- To the extent that there are different tax rates for married couples, which set up incentives for working more, increasing the size of the married population may increase total output. But then again, the welfare improving solution is not to increase the share of married couples, but removing tax benefits for married couples.
- There may be "efficiency gains" from being married. This is the only possible "real" economic argument: If, being married, and having a state-legalized marriage contract makes people very happy and productive workers, increasing the size of the married population will increase TFP. However, this is a very big if, and I am not aware of research supporting this. In contrast, happiness research seems to indicate that the impact of positive or negative events fades away quickly, and an invididuals "happiness" moves back to average levels quite quickly, even in case of losing a leg (Brickman et al, 1978; ungated)