The 2015 second-round pick assisted on Seattle's second goal of the night, which tied the game, 2-2, before the Otters scored twice in the third to take the contest.
Gropp had seven goals and 12 assists for 19 points in 16 Western Hockey League playoff games for the Thunderbirds, who captured their first WHL Championship last week.
Defenseman Sean Daywas held off the scoresheet in the Windsor Spitfires' 3-2 victory over the Saint John Sea Dogs.
The round robin tournament continues Sunday when Gropp and Seattle face Day and Windsor at 7 pm.
