G1SST is an acronym for global 1-km sea surface temperature, a product
that combines satellite data with the model output. It is provided by
the JPO ROMS (Regional Ocean Modelling System) modelling group.
See the JPL website  to learn more about the data, and see
read.g1sst documentation for an example
of downloading and plotting.
It is important not to regard G1SST data in the same category as,
amsr-class data, because the two products
differ greatly with respect to cloud cover. The satellite used by
amsr-class has the ability to sense water temperature
even if there is cloud cover, whereas
g1sst fills in cloud
gaps with model simulations. It can be helpful to consult
 for a given time, clicking and then unclicking the radio button
that turns off the model-based filling of cloud gaps.
1. JPO OurOcean Portal