DECLARE @tableLoop TABLE(id INT, sinfo INT);
DECLARE @tableResult TABLE (SINFO VARCHAR(10));
DECLARE @maxNumberOfGaps

DECLARE @loop INT;
DECLARE @maxLoop INT;
DECLARE @sinfo INT;
DECLARE @lastSinfo INT;

INSERT INTO @tableLoop(id, sinfo)
    SELECT ROW_NUMBER() OVER (ORDER BY CAST(RIGHT(SINFO, 5) AS INT)),
           CAST(RIGHT(SINFO, 5) AS INT)
    FROM student_data

SELECT @loop = 1;
SELECT @maxLoop = count(*) FROM @tableLoop;
SELECT @lastSinfo = 0;
SELECT @maxNumberOfGaps = [***desired number here***]

WHILE @loop <= @maxLoop AND @maxNumberOfGaps > 0
BEGIN
    SELECT @sinfo = SINFO FROM @talbeLoop WHERE id = @loop

    IF (@sinfo > @lastSinfo + 1) AND @lastSinfo != 0
    BEGIN
        INSERT INTO @tableResult
        VALUES('G' + @lastSinfo + 1);

        @maxNumberOfGaps = @maxNumberOfGaps - 1;
    END

    SELECT @loop = @loop + 1;
END

SELECT * FROM @tableResult;