Google provides a limited number of slots. So how you choose whether to allocate a slot to a project. Is there a priority set on each project or Is the quality of the proposals submitted for a project is considered ?
Google prepared a mentor and a contributor guide. Check more details here: Selecting Contributors and Mentors | Google Summer of Code Guides
They write:
Conceptually, the process of selecting GSoC contributors and mentors is simple. In practice, the process of prioritizing proposals and assigning mentors can be difficult and contentious.
During the GSoC contributor application period, organizations prioritize the GSoC contributor proposals, discard proposals unworthy of consideration and investigate proposals further. The organization administrator will reach out to the mentors and together they will prioritize the proposals and rank them. Then the organization administrator will submit their organization’s requested number of project “slots” on their program dashboard. Committed mentors must be assigned to all projects that the organization wants considered. Additional mentors can be added later but a minimum of one mentor must be assigned to a proposal or it can not be ranked.
GSoC contributor slot selections will be based on the rank an organization gives the proposals.