The government had announced the initial April 13 date for the vote in early February, ending months of uncertainty.
The electoral campaigns will run from midnight on April 4 through the end of April 18.
Opposition parties boycotted the last legislative elections in 2018, citing "irregularities" in the electoral lists.
This time, the opposition is preparing to challenge the ruling Union for the Republic (UNIR) party, and organized a voter registration drive.
Gnassingbe came to power in 2005 after the death of his father General Gnassingbe Eyadema, who ran the country with an iron fist for 38 years. He has been re-elected three times in elections the opposition claimed were marred by irregularities.
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