Hi Amanda,
Not all GMOs are part of the Roundup Ready® system. Roundup Ready® crops are one brand of glyphosate-tolerant crops, and have been enhanced through biotechnology to no longer be susceptible to glyphosate, a commonly used herbicide. Corn, soybean, cotton, canola, sugar beet and alfalfa are all examples of crops that have some glyphosate-tolerant varieties.
There are GMOs that are developed to resist herbicides (like Roundup Ready®), but also many others that are developed for different purposes, such as: drought resistance, insect resistance, nutritional enhancements, and food waste reduction. This article at GMOAnswers.com has more information on the types of GMOS available to farmers: https://gmoanswers.com/gmo-basics