Maybe some kind of a rule is beginning to take shape here when we sort out the examples a little bit.
The phrase containing gerek- acts as the subject of the main clause:
Gitmen gerektiği belli. It is clear that you have to go.
The phrase containing gerek- acts as the object of the main clause:
Gitmen gerektiğini söylüyor. He/she says that you have to go.
The phrase containing gerek- acts as the adverbial of the main clause:
Gitmem gerektiği zaman söyle. Tell me when I have to go.
Until here everything is simple. The problem comes when we try to use a phrase with gerek- as an attribute. These are all equivalents to English relative clauses. You can’t find –dik participle here. Instead, only participles which don’t take personal endings are used.
Bitirmem gereken ödevlerim var. I have homeworks that I have to finish.
almam gerekmiş çiçekler the flowers which I had to buy
torbasını açması gerekmiş yolcu the traveller who had to open her bag
kızının kasapla evlenmesi gerekmiş baba the father whose daugter had to marry the butcher
Actually the only example which I cannot fit into the rule is the one with future participle (which, yes, is able to take personal endings and usually acts like –dik participle).
Söylemem gerekecek şey onu üzebilir. The thing I will have to say can make him upset.
Maybe the need of expressing future is stronger here than syntactic limitations. We don’t have but –ecek- for future, do we? Just speculating.
If a word becomes adjective it can only take possesive suffixes not personal endings. And if it takes a possesive suffix it will become a noun.
Okay I accept I am a troll.