Learn Hardcore Norwegian: Spikeren sitter fast i gipsveggen.

Because spikeren is the definite form: spiker = a nail, spikeren = the nail. Norwegian often uses the definite form when talking about a specific, already-known object in the situation (e.g., the nail we’re looking at).

What does sitter fast mean, and why not just a verb like “to stick”?

Sitte fast is a very common Norwegian expression meaning to be stuck / to be firmly fixed.

sitte literally means to sit, but it’s also used about things being positioned / attached / located in a stable way.
fast = fast, firmly, stuck.
Together: sitter fast = is stuck fast / is firmly stuck.

Is sitter fast different from står fast or henger fast?

Yes, Norwegian often chooses the “position verb” based on how something is oriented:

sitte fast: attached or fixed in/onto something (nails, stickers, lids, things wedged in).
stå fast: “stands” firmly—often upright or fixed in place (can be used more abstractly too).
henge fast: hanging and stuck (something caught on a hook/branch).
For a nail in a wall, sitter fast is the most natural.

Why is it i gipsveggen and not på gipsveggen?