In my store they seemed to typically hire them from the oustide, but I'm not sure if they were actually against promoting, or if people just rarely pushed to be promoted.
We didn't have a proper guest service area, we were a store where Guest Service was just three registers at a counter near the checklanes, I know there is a term for the setup but I can't recall it any longer. The major difference was everyone was scheduled as a cashier, never as service desk. The GSA or GSTL in charge just decided which cashiers to assign to the service desk based on who was scheduled to cashier.
As a result Guest Service wasn't its own separate position. I learned guest service within a few weeks of being hired, and quickly learned the service desk better than anyone else in the store with the possible exception of the GSTL, but even he occasionally had me show him how to do things on the computers as I tended to know more about them than he did due to my propensity to experiment with them during downtime and to learn from posts on this site.
Despite that, I was never considered as a GSA initially, I trained two outside-hire GSA's to do the service desk without ever being mentioned that there was an opening beforehand either time. But despite that, as soon as I mentioned that the next time they had an opening that I'd rather be given a chance to apply for it than have it go to an outside-hire I was almost immediately given the position, and I didn't have to interview for it despite it being customary that cashiers are required to interview for GSA beforehand.
The way it was handled made me think that perhaps my store didn't so much have a preference for hiring outside GSA's, so much as they just didn't approach TM's and offer such positions to them unless the TM's explicitly mentioned being interested beforehand. More of a communication issue than anything potentially.
I haven't worked for Target since 2015 though, I eventually got fed up with Red Cards being the most important aspect of my job in the eyes of upper leadership and quit. I could be great at every other aspect of my job, but it didn't matter because being less than enthusiastic about pushing red cards on everyone made me a "problem employee".
Some of that is a bit off-topic, but the main thing is, that even stores that don't seem to promote often might not actually be opposed to doing so if they have TM's that actively express interest, they might just be less prone to suggest the idea to TM's on their own volition than other stores are.