SFM and Gmod are good in their own ways, even when it comes to posing, there are certain things you can do in Gmod that you can't in SFM, such as changing bloom and color correction in an easy to use manner, changing textures, and material settings without having to use a console command to refresh one or all materials that causes the engine to lock up for a bit
And well, there are some people that prefer the physgun because of how quicker and easier it can be, SFM is a bit more tedious and time consuming because it's more meant for precision posing and animating