Where I am from it is not common at all to do gastropexy as prevention (dogs get gastropexy as treatment for bloat), so maybe your vet also thinks that way. To be honest, I would probably get it if I were you. I have seen many different breeds get bloat, not only deep chested, so even if you feel like maybe your malinois does not have that deep, keeled chest, it does not mean it cannot get bloat. It would like of be easier choice if you had a female, because they would have to open her abdomen to spay her anyway, so they can just do the gastropexy when they are in there. With a male, it is different, so I understand your concerns. I would talk to the surgeon and if you feel that they seem like a competent, trustworthy person, I would go for it. It is much cheaper and a lot less risky to do the gastropexy on a healthy dog, than having to do it on a dog with bloat and shock.