I would start with the free Melda bundle (includes Flanger and Phaser): MFreeFXBundle | MeldaProduction
Otherwise, there is a great thread in this forum:
With the huge number/range of plugins that users of Gig Performer have in their gig files, I think the likelihood is higher that the issue is with the plugin. If they are no longer being actively developed, then there is zero chance that the GP devs could work with the plugin dev to look into it.
I recommend when you hit issues with these old/free plugins that you simply move on immediately to something else - much less hassle in the long run.