That depends on how you build your French sentence but the most straightforward translation would be:

C’est ennuyeux

The adjective needs to agree with the neutral pronoun ce (c’) which is technically always masculine and singular, regardless of what it refers to.

Not sure about the English sentence, but the French one would be ambiguous. What is ennuyeux might be the biology class but also the fact Gabrielle is attending that class.

To sort it out, you might write:

Gabrielle a biologie, c’est une matière ennuyeuse. (boring)

and for the alternate meaning:

C’est ennuyeux, Gabrielle a biologie. (ennuyeux = troublesome/annoying here)

“ennuyeux” here doesn’t refer to “biology” but to the fact that Gabrielle has a biology lesson. This notion is not feminine, thus you can’t use the feminine for “ennuyeux”.

“It is” will be translated by “C’est”, “c'” being the shortened form of “ça” which refers to the whole previous sentence (and is masculine).