Are the GB & GBC 1 & 2MHz systems or 4 & 8MHz systems ?

This depends on whether you are talking about clock cycles or machine cycles. Nintendo specifications list machine cycles. Many internet document specifications list clock cycles.

The GB & GBC can operate at ~1 million machine cycles/sec or ~4 million clock cycles/sec. The GBC in double-speed mode can operate at ~2 million machine cycles/sec or ~8 million clock cycles/sec.

The simplest assembly instruction in the GB & GBC is a NOP. It takes 1 machine cycle or 4 clock cycles for this instruction to execute.