On gap-fill questions, one thing first: if there is any word or sentence in the question you do not understand, write that question off. If you have not understood the question, you will certainly not find the answer.
The usual advice, and why I do not give it
People commonly say: underline the keywords and the key ideas in each sentence.
I do not recommend it. Underlining gives you the feeling of doing something useful. But you have not understood the passage any better and you have not read any more carefully. Your eye has seen the word; your brain has not necessarily processed the sentence. It does not work.
What to do instead
Ask yourself two questions:
What kind of word goes here? Noun, verb, adjective, a number?
What might the answer be? Have a guess.
Why this is better
Sitting and working out what word class you need, and then predicting the answer, proves that you have read the sentence closely. You cannot do it without understanding it.
To be able to say "I think this is a noun" or "I think this is a verb", you are forced to understand the question thoroughly. There is no way to fake it.
That is the whole difference. You can underline without thinking. You cannot predict a word class without thinking.