The first GMO? The first ever genetically modified crop production was in 1994, the first being the GM Flavr Savr tomato in the US.
What have they done? Improved weed and insect control from GM crops has resulted in major environmental benefits through fewer chemical applications.
The adoption of GM crops has resulted in economic benefits for farmers, reduced chemical use in the environment and most importantly a reduction of numerous cases of pesticide poisoning.
These benefits are not only gained by developed nations but have had significant impacts in developing nations, helping agricultural regions leap from agrarian to modern practices (Smyth).

Crops engineered to produce insecticides produce toxins that are not only harmful to pests but other insects such as butterflies, moths and insect pollinators.
Where they’re grown: GM crops occupy large surface areas and are linked to intensive monoculture systems that wipe out other crop and ecosystems.
Little is understood yet about the health effects of GMOs, but recent studies have shown animals fed with GM-containing feed can develop health problems.
Are they ok to eat? The majority of GMO crops are not meant for human foods, but rather for animal foods and biofuels.
GMOs have not increased productivity; data from the USA’s Department of Agriculture shows that there has been no recorded increase in soya and corn yields since the introduction of GMOs.

Amy Dean, a physician specializing in internal medicine, worries over the increasing number of cases of autoimmune diseases like multiple sclerosis, inflammatory bowel disease and arthritis. The immune system is set up to fight off foreign materials, and it’s believed that the new proteins created by GMOs may, in effect, make the body react to itself.
Health experts seem to agree on one risk: the potential for GMO-related food allergies. In 1996, researchers inserted what turned out to be a highly allergenic protein from a Brazil nut into soybeans. The soybean never made it to market, but it did raise concerns. Some GMO critics blame these modified crops for the increase in food allergies in children, which jumped 18 percent from 1997 to 2007 (Young).

Citations:

“Why We Are against GMOs .” Slow Food International,
www.slowfood.com/what-we-do/themes/gmos/why-we-are-against-gmos/.

Young, Allison. “GMOs: Friend or Foe?” Natural Health, vol. 42, no. 4, Apr. 2012, pp. 46–50.

Smyth, Stuart. “25 Years of GMO Crops: Economic, Environmental and Human Health
Benefits.” Genetic Literacy Project, 9 Jan. 2019