Would there be a force carrier particle in the unified field theory? So from what I understand, each fundamental force (with the exception of gravity unless you count the hypothetical graviton) has a force carrier particle that mediates the force. Does that mean if the grand unified theory, where each force unites as one at high energy, is true then there would be a singular force carrier particle that would have had decay into the particles we have today? If so, does that mean the electroweak force has a force carrier particle (I know the $Z$ boson plays a role with the electroweak force theory but I have to admit I'm not quite sure how after reading about it)?
 A: Firstly, let me notice that a Grand Unified Theory (or GUT, for short) is a hypothetical unification of the Standard Model interaction, but it is not necessary that it corresponds to reality. There are GUT models, but it is not certain as of now that any of them actually describe reality (although they are built so that they recover the Standard Model and many haven't been ruled out yet).
That being said, yes, there are force carrier particles (plural) carrying the interaction of the "grand unified force", just as the Standard Model has carriers for its own interactions. In the Standard Model, we have

*

*8 gluons carrying the strong interactions;

*the photon carrying the electromagnetic interaction;

*the $W^+$, $W^-$, and $Z$ bosons carrying the weak interactions.

Notice that it is not a single carrier per interaction. The appropriate number depends on the details of the interaction. More technically, each carrier is associated to a generator of the gauge group. Since the number of carriers depends on the details, it also follows that the number of carriers on a GUT depends on the specific GUT being considered. As noted in the comments, the SU(5) GUT has 24 carriers. Out of these 24, 12 will eventually be identified as those of the Standard Model (the 12 ones I listed above), while the other 12 are bosons not present in the Standard Model, which lead to new predictions, such as proton decay. Other models have other numbers of carriers. The SO(10) GUT, for example, has 45 (24 of which are the ones from SU(5)).
If there is a correct GUT, the explanation for why we don't see these extra particles at the LHC, for example, is that they are way heavier than the ones we've seen so far. ELectroweak interactions occur at the scale of $10^2$ GeV, while the scale in which we expect grand unification to happen is close to $10^{15}$ GeV, meaning present-day experiments can't gather enough energy to see these sorts of particles.
In short, there are new carriers (plural) in GUTs, in addition to those of the Standard Model. The weak interactions do have carriers (plural), those being the $W^+$, $W^-$, and $Z$ bosons.
As a final remark, it is worth pointing out that GUTs do not attempt to describe gravitation and hence are not attempts at a theory of quantum gravity, much less at a theory of everything. While your post suggests that you are aware of this, I believe it is good to reinforce this point for other readers of this post.
