Where did you look? The dictionaries that I have at hand list gorchmynnol as imperative. (Strictly speaking this is an adjective, so the term you are looking for is yn y modd gorchmynnol, literally in the imperative mood, although I have also heard it simply as y gorchmynnol)
Hendrik, may I ask: what dictionaries do you use? I’ve been looking for a decent dictionary for a while. I have Y Geiriadur Mawr at home but I find it quite limited - the name is rather misleading as it’s a tiny dictionary!
Sure. I have an android smartphone, on which I use two digital dictionaries: “Ap Geiriaduron” and “Geiriadur Prifysgol Cymru” (GPC for short).
For a printed dictionary, I use @garethrking 's “Modern Welsh Dictionary”. It goes above and beyond a dictionary’s call of duty of merely listing the meaning in the target language in that it also gives immensely valuable additional information of how to use certain trickier words.