Technically, the 1974 Graduale Romanum is a private edition of the Abbey of Solesmes. It follows the order of chants laid out in the 1970 Ordo cantus missae (editio typica). The latter refers only to the 1908 Graduale Romanum (editio typica), which bears no rhythmic markings, like the ictus, episema or dot. Even though private, the 1974 edition was the only Graduale composed after the promulgation of the OCM, until in 2011 the Graduale Novum was published (also a private edition, by the way, and the result of new interpretations of the manuscripts). That's how for decades, the 1974 edition was the only chant book singers could rely on for the Ordinary Form, and thus acted de facto as an official chant book, even though it was not. I even suspect that, whenever the GIRM refers to the Graduale (#48, #87), it actually means the 1908 Graduale, adapted to the Ordinary Form according the OCM, although it never states this explicitely.

For the 1974 Graduale Romanum, Solesmes rearranged the original engravings from its 1962 Graduale Romanum (and made some small adjustments too). This 1962 edition was also a private one, modelled after the typical edition of 1908, and included the rhythmic signs designed and added by Solesmes. That's how the rhythmic signs ended up in the 1974 Graduale.

These rhythmic signs gave directions for the execution of the chants, based on a certain interpretation of the original manuscripts and a private theory of rhythm in Gregorian chant developed at Solesmes. In its modern editions of chant books (after 1980), Solesmes has abandoned these rhythmic signs once again, leaving it now to the singers to employ their own ideas and interpretation of the chants.

That means that the ictus markings in the 1974 Graduale Romanum are in no way obligatory. If you would like to follow the old Solesmes rhythmic interpretation, feel free to use them. If not, just ignore them.