momentum conservation and gluons The process is the following:
$$e^-e^+ \rightarrow photon \rightarrow quark + antiquark$$
Regarding the momentum conservation law, how come we have a photon of spin 1 and at the end some meson with spin 0? 
Are gluons "behind this"? If this is correct, at which point are they radiated? From quarks? Or? Is this photon a virtual photon or not? I'm a bit confused here.
 A: Ignore the gluon for the moment


Regarding the momentum conservation law, how come we have a photon of spin 1 and at the end some meson with spin 0?

First of all spins are angular momentum  not momentum. Secondly the two quarks have a spin 1/2 which will add to  either 0 or 1, and 1 conserves the angular momentum at the vertex.
All intermediate lines in Feynman diagrams describe a virtual particle, in this case the line describes a particle with the quantum numbers of a photon, but not the mass. It is off mass shell and by definition virtual.

Are gluons "behind this"? 

No. Quarks couple to electromagnetic fields because they are charged. The vertex is electromagnetic. Quarks couple to gluons because they have color charge., and actually no free quarks exist because the attraction created by the virtual gluons ties quarks up into mesons and baryons; this can happen at a later stage as the green gluon shows here, which should end up to another quark antiquark line to make maybe a rho-meson (spin one, used to be called vector meson dominance).
