グレシプ and #GraySheep are the ones the official account uses to share info and make announcements, but you are free to use them to post merch, MVs impressions, etc (as long as it's not paid content). Posting fanarts seems to be okay too as long as they do not portray any ships: it is common knowledge in JP fan-spaces and sometimes seiyuus and those behind a project look up their works on social media as well. There are also all kind of fans, so it is better to leave ships to their own tags.

グレシプ_FA for fanarts. However, it seems to be still a pretty new hashtag, with little to no content. The ship warning applies to this hashtag as well. Some artists have also been using #グレシプFA .

バスカ (BASKU) and #GOAT are the official hashtags for each organization.

Since Gray Sheep is fairly new, there aren't many hashtags worthy of attention besides these at the moment. Will update this post when/if we get more!