Below is a list of projects that are based on gem5, are extensions of gem5, or use gem5.

MV5

MV5 is a reconfigurable simulator for heterogeneous multicore architectures. It is based on M5v2.0 beta 4.
Typical usage: simulating data-parallel applications on SIMT cores that operate over directory-based cache hierarchies. You can also add out-of-order cores to have a heterogeneous system, and all different types of cores can operate under the same address space through the same cache hierarchy.
Research projects based on MV5 have been published in ISCA'10, ICCD'09, and IPDPS'10.

Single-Instruction, Multiple-Threads (SIMT) cores
Directory-based Coherence Cache: MESI/MSI. (Not based on gems/ruby)
Interconnect: Fully connected and 2D Mesh. (Not based on gems/ruby)
Threading API/library in system emulation mode (No support for full-system simulation. A benchmark suite using the thread API is provided)

gem5-gpu

Merges 2 popular simulators: gem5 and gpgpu-sim
Simulates CPUs, GPUs, and the interactions between them
Models a flexible memory system with support for heterogeneous processors and coherence
Supports full-system simulation through GPU driver emulation