The NEXUS endorsement is for expedited entry into Canada. But without it, Global Entry still works for expedited re-entry back into the US. As for Mexico, you

The NEXUS endorsement is for expedited entry into Canada. But without it, Global Entry still works for expedited re-entry back into the US. As for Mexico, you just need to register your vehicle on the GOES/TTP portal to use your Global Entry card in the SENTRI lanes. (The vehicle inspection requirement was dropped recently.)