According to one Western source Russia was subsidizing 10–15 percent of Belarus' GDP around 2013 or so. Another source source quantifies that differently as "Between 2005 and 2015, Moscow pumped $106 billion into the Belarus economy, according to the International Monetary Fund (IMF)." Which seems a considerable sum.
Were Western sanctions on Russia imposed around 2014 relevant in the reduction of Belarus' GDP? It does seem noticeable in that regard, e.g. relative to Romania's, in the same time frame (graph pointed out to me by Tadeusz Kopec)
On the other hand, it does look like the reduction in Belarus GDP was around 2016 so I'm not sure if the effect was delayed or even caused by something else entirely. So did Russia reduce its aid/subsidies to Belarus after 2014?