Thank you so much.
I think they are very much of the type of group who have a specific position and then cherry pick evidence to support that. Any study that confirms their viewpoint is quoted, even ones that have been consistently debunked. Studies that don't support their viewpoint are either ignored or they lie about their findings.
For example, this is how they summarise one of the twin studies that I mentioned before:
This lack of a durable biological underpinning of a transgender identity is demonstrated best by studies involving identical twins, which show that only 28% of the twin pairs will have both twins developing a trans identity, despite having identical genes.
They fail to mention that those studies showed that the degree of concordance in identical twins was significantly higher than in non-identical twins. The studies have some pretty significant limitations, such as very small sample sizes, and one which recruited participants from an online forum... hardly rigorous or safe from bias. It is absolutely valid to question the strength of that evidence.
What is not valid, however, is to use them to bolster the claim that there is no genetic component. Most conditions occur as a result of a complex interaction between multiple genes, the environment, social factors etc. For example, genetics are known to be a significant factor in the development of type 1 diabetes. However, it is a multifactorial disease - several different genes have been identified and display incomplete penetrance, and environmental factors (e.g. coxsackievirus infection) also appear to be important. As such, when you look at twin studies the concordance rate for type 1 diabetes is never 100%. It is actually very close to the concordance rate displayed in the twin studies referenced by Genspect.
They also have outright falsehoods on their website. For example, they claim that before the introduction of the first puberty blocker trial in the UK (2011), only people aged older than '30-35' were allowed to transition. That is patently false.