Does these (in the quotation) sound natural?

"Are safe" is correct here.

(The claim is: they were safe in the past, they are safe now, and they will continue to be safe. So choose the present tense.)

Has the expert told THE truth? (A general term, meaning truthfulness in all he was saying. Don't confuse it with "a truism".)

Do these, or does this....

modified food) IS safe

told the truth?

No. See above.

. . . that GMF (genetically modified food) are safe and that they have been eating GMF safely for twenty years.

You can't say "food are safe". This has already been pointed out. You can't say "snow are cold" or "fire are hot" for the same reason.

Yes, it should be "is safe."

I added another "that" to the second clause of the sentence .

have been reassured
that GMF (genetically modified food) is safe
and
that they have been eating GMF safely for twenty years.

But you can say that GMF (genetically modified foods) are safe.

The plural works equally well here. But you must be consistent; the verb must agree with the subject.

Thank you guys.

Now the edited version is:

In the past two decades, (almost all) Americans have been reassured that GMF (genetically modified food) is safe and they have been eating GMF safely for twenty years.
Has the expert told the truth? Or has he told a lie?

Three questions remain:

1) "In the past two decades" looks a bit awkward to me. But how to improve?
2) "for twenty years" looks not very well there.
3) I changed "has he lied" into "has he told a lie", in an effort to look better matched with "told the truth." But I am not sure whether it got improved.

Right now you have a simple compound sentence:

Americans have been reassured that GMF (genetically modified food) is safe
and
they have been eating GMF safely for twenty years.

If you want to say that they have been assured TWO things, then you need another "that" in the second clause (and that they have been eating . . . .)

++++++++++++++++++++++++++++++++++++++++++

"In the past two decades" is redundant. You already talk about 20 years later in the sentence.

Why say ("almost all?) Why not use many or most?

You could try:
Most Americans have been reassured that GMF - genetically modified food - is safe and should realize that they have been eating GMF safely for twenty years.