So for those who don't know, if you Terastallize, moves of your Tera Type which are not priority or multi-hit, and which are under 60 BP, become 60 BP. This goes, for instance, for Trailblaze and Flame Charge, which is why Koraidon's Flame Charge under sun hits so hard with Tera Fire (it's effectively 90 BP STAB).
However, priority is not affected. Grassy Glide isn't necessarily priority though, which opens up for questions: does the move's BP increase, does not increase, or does it only increase outside of Grassy Terrain?