GnRH agonists used to block puberty include leuprorelin (Lupron) and triptorelin (Trelstar). There are multiple studies pointing to GnRHas lowering IQ and negatively impacting brain function in general.
https://www.frontiersin.org/articles/10.3389/fpsyg.2017.00044/full#B6
Gonadotropin releasing hormone agonists (GnRHas) have been found to impair memory in adults, so the study by Wojniusz et al. (2016) on the possible cognitive effects of these drugs on children treated for idiopathic central precocious puberty (CPP) represents an important contribution to research in this area . . . Recent findings that GnRHas increase depression symptoms (Macoveanu et al., 2016) and slow reaction time (Stenbæk et al., 2016) in healthy women, and reduce long-term spatial memory in sheep (Hough et al., 2017) underline the importance of the research that Wojniusz et al. (2016) have undertaken. Girls treated for CPP with triptorelin acetate were tested with the short form Wechsler Intelligence Scale for Children III.
The findings of Wojniusz et al. (2016) can be compared with those of a 2001 study in which 25 children treated for early puberty with triptorelin acetate were tested with the short form Wechsler Intelligence Scale for Children (Mul et al., 2001). In this longitudinal study, children took the IQ test before treatment and again after 2 years of treatment.
It was found that their IQ dropped 7 points from 100 to 93. With 25 treated participants, this 7 point drop was significant (p = 0.002).
https://www.ncbi.nlm.nih.gov/pubmed?Db=pubmed&Cmd=ShowDetailView&TermToSearch=11683207
Single person study of effects of puberty blockers on 12-year-old shows IQ drop of 9 points in 15 months
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5694455/
Staphorsius et al. (2015) conducted a study in a GD adolescent group under hormonal suppression to investigate the impact of pubertal suppression on executive function (EF). They compared GD adolescents under GnRHa treatment to GD adolescents undergoing physiological puberty and compared them to male and female control groups. They used the Tower of London test and found a negative impact of pubertal suppression on EF. However, they also associate this outcome with a lower IQ before GnRHa treatment.
Additionally, a global IQ decrease (WISC-III) was reported in a longitudinal follow-up of girls with central precocious puberty (Schuerger and Witt, 1989) treated with GnRHa.
Finally, a third study correlated verbal skill impairment to pubertal suppression in a GD group (Costa et al., 2015).