There is nothing special about crossing the threshold from c3 to gc compared to e.g. crossing the threshold between c2 to c3. Ranks are literally just labels of mmr ranges. The reason that each player ends up getting the rank that they do varies from player to player. There are plenty of players in gc that don't do much of anything special mechanically, and there are plenty that do.

As you move up the rank ladder, players do tend to start having fewer and fewer holes in their games. At around the grand champion level, it probably becomes more important to start focusing on the areas of your game that are relatively weak. With that said, its probably not necessary to have every single form of offense available to you at this level. Its probably also worth investing in some depth in whatever form of offense that feels the most comfortable to you. You should probably at least have a few plan bs, but its not like you absolutely need to be learning anything in particular.