For a starter pack gnosis, it depends. Myrians (and half-Myrians if raised in Taloba, I believe) can get Myri's gnosis by just applying at the HD with a simple reuqest (e.g. 'I'd like my Myrian to be given Myri's gnosis, please'!). It's the same process for Svefra's and Laviku's gnosis as well. So if it's Myri's gnosis that you're hoping to get, you technically don't need to write any threads worshipping her to get the gnosis
If you want a starter pack gnosis from any other deity (other than Myri, basically), you also apply at the HD before reaching 50 posts. But instead of a simple request, you'll need to post a story about how your PC attracted the God's attention, etc. These would vary in length, but the more detailed, the better! I don't *think* there's a particular post/word limit, but simply saying 'One day, Bob stroked a kitten and Caiyha appeared and gave him her gnosis' would obviously not be good enough! It depends on the God in question, I suppose. Caiyha, for example, appreciates defending her creations from harmless violence, whereas Rhysol prefers an individual to purposefully create chaos in the world for jollies. The thread you'd post in the HD would simply tell a story centring around the relative 'thing' or act a God likes.
Once you surpass 50 posts, or if you want a level 1 gnosis from another God after getting your initial starter park gnosis, you'll have to contact an DST or above to discuss why you think your character should get that gnosis, and then the two of you would do a modded thread together. These are the gnosis request that usually require more effort, as by that point your PC has developed a certain degree. So it might be worth doing a few threads (say.... 4/5?) PRIOR to contacting a DST where your PC learns about, worships, discusses, or does some act that the God might like. These can vary in word length, I'd imagination and again depends on the content. A thread where your PC learns about a God might only actually involve 1 post where your character actually learns about the deity themselves, but threads where they carry out the God's work would obviously be more, but again depends on the storyline and plot you use.
Hope this helps!