PROVIDENCE, R.I. (AP) _ The president and chief operating officer of Gtech Corp., Deward F. Manzer, resigned from the lottery equipment and services company without explanation today.

Manzer, 49, had been president since December 1987. The announcement said only that he had resigned ''to pursue other professional interests.''

Neither Manzer nor Guy B. Snowden, Gtech chairman and chief executive, returned telephone calls to the Associated Press.

The announcement said Snowden would assume Manzer's responsibilities, but it was not clear if he also would assume the title of president.

The resignation apparently was discussed at a board meeting Thursday, but no other details were immediately available.

Gtech, which had gone through several quarters of shrinking profits and rising stockholder complaints during Manzer's tenure, recently reported its most profitable quarter in two years, earning $2.69 million, of 28 cents per share, on revenue of $43.7 million for the third quarter ended Nov. 26.

Earnings for the same quarter in 1987 were $2.51 million, or 26 cents per share.