If this was TRUE, then the opposition would have pretty solid cause for complaints against Lukashenko's regime.
It would have very simple for anyone in Belarus to do a Trace Route (tracert) and confirm for SURE what was going on. Most advanced computer/IT people would know how to do it. Is a tracert available?
Otherwise it's just general rumour spreading. And how popular are these sites anyway? Wouldn't it make more sense to block the main Russian speaking websites? For example Mail.ru rather than Gmail.
In China, for example, there is no doubt that these things are happening; the users can trace their packets and and their requests get redirected long before they get to the blocked site. They get re-directed to a government site with an sign of an animated policeman and some text. It's clear as daylight what's going on. DDosing sounds really amateurish; likewise slamming a block on some randomly selected sites for one day only.
Belarus is not generally considered to be a country that censors internet usage.
Any pro-Lukashenko hackers/security people can only do two things: Block access to the opposition sites at the ISPs (which you can still get around with a proxy or VPN), or DDOS the sites (for that, you need your own botnet which usually only cybercriminals have.).
Neither action makes any logical sense for them to be doing on the election day in particular, but not otherwise.
The sites are normally accessible to Belarussians. Blocking it on the election day would simply be a bad reflection on them and achieve no benefit.