  Making general particle transport simulation for high-energy physics (HEP)
single-instruction-multiple-thread (SIMT) friendly, to take advantage of
accelerator hardware, is an important alternative for boosting the throughput
of simulation applications. To date, this challenge is not yet resolved, due to
difficulties in mapping the complexity of Geant4 components and workflow to the
massive parallelism features exposed by graphics processing units (GPU). The
AdePT project is one of the R\&D initiatives tackling this limitation and
exploring GPUs as potential accelerators for offloading some part of the CPU
simulation workload. Our main target is to implement a complete electromagnetic
shower demonstrator working on the GPU. The project is the first to create a
full prototype of a realistic electron, positron, and gamma electromagnetic
shower simulation on GPU, implemented as either a standalone application or as
an extension of the standard Geant4 CPU workflow. Our prototype currently
provides a platform to explore many optimisations and different approaches. We
present the most recent results and initial conclusions of our work, using both
a standalone GPU performance analysis and a first implementation of a hybrid
workflow based on Geant4 on the CPU and AdePT on the GPU.
