my cat has black stuff by his gums?

No one here can diagnose your cat remotely. Even a vet reading your question could not determine the cause of the problem without examining your cat and it is highly unlikely that there are any vets here and anyone saying they are a vet is likely lying.

Any "diagnosis" you get here will be no better than a guess.

Your cat needs to be examined by a vet.

A basic vet office visit exam should cost between $35 and $55 dollars. That is not much for the type of exam it will buy you.

The vet should do a full exam of the cat, nose to tail, and look at any specific problems.

You can tell the vet not do anything that will add to the bill without your prior approval.

Do a Google search on :

"low cost" vet cats city state

Put the words low cost in double quotes as I show them and replace city and state with your city and state.

You may find some low cost vets in your area.

Check with the Humane Society, the ASPCA (RSPCA if you are in the UK), and the APA (Animal Protective Association) and see what they charge for a basic exam.

There is a charitable vet service in the UK called PDSA, here is their eligibility page http://www.pdsa.org.uk/eligibility.html