How can you research the effects until a few years have gone by? Or do you think consuming GMO would be an immediate noticeable effect?
I am sure they checked for exactly that, as one of the first steps of testing!
If you feed something to an animal, and the animal dies immediately (or in a few days or weeks), then of course you don't want to give that substance to a person.
You can also test on fast-growing animals, because they tend to show problems more quickly than slow-growing animals. For example, if a mouse is fully grown in 2 months, you can easily check if the GMO food makes it grow much slower or faster than other foods.
Yes, longer-term effects take longer to show up, and effects in longer-lived creatures (like people) can also take longer to show up.
But there also comes a point when it's not worth looking further. For example, if eating it for 150 years would cause a problem in people, that does not matter because people do not live 150 years.
Also, it's harder to tell the cause for long-term effects. If a child eats GMO food, uses a computer, goes to school, rides in a car, and wears a certain kind of clothing, and then they develop a problem at age 50: it might be very difficult to determine which of those things caused the problem, or whether the problem was caused by something else yet.
I agree that GMOs have not been around long enough to show really long term effects-- but any effects that would show in the short or medium term should have been seen by now.
GMOs clearly do have some benefits, and so far they have not been shown to have serious bad effects. Considering the amount of time they have already been studied and used, it seems unlikely that any other BIG effects will show up. Small ones, maybe. But not big ones.
There are many things in life that we use even though we know they cause bad effects, because we think the trade-off is worth it. For example, cars can get in accidents that injure or kill people, but we keep using them. People can get injured while playing sports, but many people do that too. Eating too much and not getting enough exercise is bad for us in many ways, but many of us do those things too. (and so on, and on, and on.)
If you want to avoid GMOs as a personal choice, that is fine. But if you are trying to convince other people that there are good reasons to avoid GMOs, I do not think the evidence supports that.