The definition of "Student" in the VRC Game Manual is as follows:
Student - A person is considered a Student if he or she meets both of the following criteria:
Anyone who is earning or has earned credit toward a high school diploma/certificate or its equivalent during the six (6) months preceding the VEX Robotics World Championship. Courses earning credits leading up to high school would satisfy this requirement.
Anyone born after May 1, 2000 (i.e. who will be 19 or younger at VEX Worlds 2020). Eligibility may also be granted based on a disability that has delayed education by at least one year.
We will assume that the hypothetical individual in question satisfies criteria 2.
Although the definition does not explicitly include the words "provided that the student has not already achieved a high school diploma", this is supposed to be implied.
The intent of the "earning credit toward a high school diploma" verbiage in criteria 1 is to permit eligibility for both traditional and non-traditional students. This could mean those attending homeschool, who are enrolled in remote learning programs, or who are otherwise unable to participate within a "standard" school system.
To be clear: Graduating from high school, and then re-enrolling in high-school-equivalent courses for the sole purpose of competing in VRC is not within the intent of the rules, and this student would not be considered eligible to compete as a High School Student.
Now, because the question is being asked by a VEX U team, and we have established above that this hypothetical individual is not eligible for VRC HS, we should also look at VEX U rule VUG6:
<VUG6> Each Robot is allowed up to three (3) Drive Team Members, as stated in <G6>.
a. Drive Team Members MUST be post-secondary school individuals. Any matriculated individual enrolled in post-secondary school is eligible to be a Drive Team Member.
b. Professionals not enrolled in post-secondary education are not eligible to be Drive Team Members or participate on a VEX U Team.
c. Students that are dual-enrolled in both a secondary school and in post-secondary courses are not eligible to be Drive Team Members or participate on a VEX U Team.
As you noted, a GED in the US is typically considered equivalent to a secondary school diploma, not post-secondary school. Therefore, an individual pursuing a GED would not be considered "matriculated" or "enrolled in post-secondary education". Therefore, they would not be eligible to participate on a VEX U team.
The VEX Robotics Competition is intended to be an educational competition for students enrolled in educational institutions. For an individual who is not enrolled in formal education but still wishes to participate, we would advise mentoring a VRC team or volunteering at local events until they become eligible again.