I assume by “TV build” you’re running LW, SI and CR? If you sim and only change 1 talent, it’s not going to show the full benefit of it.
I also hope you’re running the Slick Ice legendary when you sim TV, because higher IV uptime means you gain more benefit from that.
From all accounts in both sims and from the top Mages in the world, GS is a bad talent to take. As the poster above says, it’s neutral dps at best and a loss at worst. It has no synergy with any legendary or conduits.
As for the sims, they only cast GS after Flurry because it’s a damage loss otherwise. GS has to crit to make it worth the cast time, and the only way to ensure that is Flurry and a decent crit rating.