Hi! Firstly, I want to point out that you seem to have a slight confusion as regards the correct structure of interrogatives or sentences that ask questions, for which please consider the following:
This phrase is grammatically correct
Name of the part of speech each word in the above statement belongs to:
This (pointing pronoun) phrase (noun) is (helping verb) grammatically (adverb) correct(adjective)
Breaking the sentence into meaningful parts:
1. This phrase is
2. grammatically correct.
1. Pronoun - helping verb - article
2. adverb - adjective
Now, please note that in order to change a statement (like one we have above) into a question, only the part 1 receives the change, leaving the part 2 as it is. Therefore, the helping verb ('is' in the above case) is placed in the beginning, followed by the pronoun (this) and then the noun (phrase) leaving the rest part as it is, to turn the statement into a question. As a result, we get the following question:
'Is this phrase' 'grammatically correct' ?
Moreover, I would draw your attention to the fact that the group of words your question is about is not a phrase but should rather be called sentence.
Phrase: a group of words that doesn't convey a complete thought, for example: yellow cab (the two words 'yellow cab' alone fail to convey a complete thought and require more words that would say something about the yellow cab, necessitating the existence of a sentence) So,
Sentence: a group of words that conveys a meaningful thought, for example: Since I don't own a car, I will have to take a yellow cab to the school.
Therefore, the group of words your question is about should be referred to as a sentence as opposed to phrase.
Now your answer in my opinion (though I agree to the 'flawless' suggestions by my above friends) :
The use of Genetically Modified Food has been a controversial issue 'as to' whether or 'regarding' whether it provides benefits or not, for the last quarter century.