Bangladesh got their tour of Sri Lanka off to a disastrous start when they crashed to a 10-wicket defeat in a 50-over warm-up match in Moratuwa.
Bangladesh had no answer for opener Gunawardene
Openers Avishka Gunawardene and Upul Tharanga cracked unbeaten centuries as a Sri Lanka XI scored 239 without loss after restricting Bangladesh to 235-8.
Rajen Saleh top-scored for Bangladesh with 53, while Mohammad Ashraful added 37 and skipper Habibul Bashar made 31.
Bangladesh play their first one-day international in Colombo on Wednesday.
They then take on Sri Lanka in two day-night games - on 2 and 4 September.
The opening Test starts on 12 September, with the second eight days later.