When something sounds weird there must be a reason for it. (I don´t need the reason but it is just interesting.)

There is one reason that is -dik doesn´t make "gerek" an adjective. It doesn´t make every verbs adjective. It chooses the one it wants

It is true.

Ex: gerektik, koştuk, güldük, etc... don´t mean anything as adjectives but bildik, alışıldık, görülmedik make sense.

Quote:gokuyum

It chooses the one it wants

I find myself often thinking this way. Just like words and linguistic elements have a character. Some are nice and flexible: whichever environment they go to they are welcomed. Others are like angry old men who smell bad and have their stubborn habits. I think there is even a basis for this kind of symbolics. Every piece of language has its history and it is there whether we recognize it or not.

Another thing about gerek- is the division of duties between the verb forms and the adjective gerek|li. Maybe gerekli doesn´t want to give the participle forms a chance to breethe.

Gitmen gerektiğini söylüyor.

gerek-dik-i-n-i

Now we see "gerek" and "dik" together. But here "-dik" with "-i" makes it a noun not adjective.  So if we add -diği to gerek we have a noun.

Ex:

1)Gitmen gerektiği belli. It is clear that you have to go.

Gitmen gerektiği is the subject of the sentence.

2)Gitmen gerektiğini söylüyor. He/she says that you have to go.

Gitmen gerektiğini is the object of the sentence.

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.

Here are the suffixes that makes verb nouns:

-an, -ası-, -mez, -ar, -dik,-ecek,-miş

Sorry, I used the wrong term maybe.

Ok, scalpel, the simpler the better. Just one thing:

 Sadly, you can find all sorts of wrong things on the internet. Just because someone wrote it, doesn´t mean it is right!

Why do you sabotage me scalpel?