Please note, this article is written to expand a lay person’s knowledge of their body. The better you understand your body as an athlete, the better you will be able to look after it.
The gastrocnemius muscle is the muscle that bulges out on both sides of the upper calf. 
The soleus muscle lies beneath the gastrocnemius muscle. Have a look at https://sbrsport.me/2014/06/13/soleus-know-your-muscles-series/ for details on the soleus.
The muscle originates at the posterior or rear part of the femur. It runs down approximately midway through the lower leg where it attaches to the soleus and gastrocnemius tendon. These attach to the calcaneus or heel bone in the form of the Achilles tendon. The muscle plantar flexes the foot. When you press your foot down you are using the gastrocnemius. The muscle is a biarticulate muscle which means that it runs over two joints. The soleus muscle consists primarily of slow twitch muscle fibres, while the gastrocnemius has a far higher percentage of fast twitch fibres. This puts the gastrocnemius at more risk when doing speed work.
Massage
The muscle has two heads, an outer or lateral one and an inner or medial one. Most athletes will find that they have a propensity to overwork and thus strain one of the two sides. In our experience, more runners end up straining the lateral or outside head. Calf pain needs to be treated seriously as it can linger for a period of time.
https://sbrsport.me/2014/02/28/sport-massage-at-sbr-sport/
Stretching
The ‘wall push-up’ is the most well-known stretch. Balance your weight equally between 
your supporting leg and the leg undergoing the stretch. Slowly drop your heel until the stretch takes place. The stretch must be gentle at a pain threshold of between 4 or 5 out of 10. There is a small danger of stretching to the point where you strain or even rupture the Achilles tendon.
Important points:
Remember to warm up slowly.
The muscle can get strained when forcefully pushing off from your foot. This would include speed work for runners, jumping, coming out of a deep squat etc.
Minimalistic shoes will put this area under more strain.
The muscle can get strained from excessive pronation.
The muscle can get sore from running in older shoes that have lost their shock absorption abilities.
People with plantar fasciitis do well to keep the muscle stretched.
Hope this helps.
Regards,
Mike Roscoe.
