A coalition of 14 opposition parties has said it will not take part, citing “irregularities” in preparations for the vote and calling for an overhaul of the electoral commission.
It has also called for more transparency and constitutional reforms to limit the number of presidential terms.
Gnassingbe has been in power since 2005 after succeeding his father, General Gnassingbe Eyadema, who ruled the country with an iron first for 38 years.
Ahead of the December 20 parliamentary elections, the opposition called for 10 days of protests to halt the ballot while urging supporters to follow them in a boycott.
Earlier this month, at least four people were killed when opposition supporters clashed with the security forces in the capital Lome and several other cities across the country. The opposition put the figure at six dead.
Such violence is unusual in this tiny west African country.
“We have always said no to these elections, we will do what we can so that they don’t take place,” coalition co-ordinator Brigitte Adjamagbo-Johnson said this week.
“The government does what it wants in the country,” a source within the Panafrican National Party (PNP) told AFP.