I looked at other French names with known pronunciations that had similar sounds or letters.
Jacqueline and
Marguerite have the "ue" that is pronounced with an ə sound (like the a in about). Geneviève has the "ièv" that is pronounced YEV as well as the beginning "zh" sound (like in vision).
Ambre and
Aurore have the "re" that is pronounced er (like in biter or fighter).
So Guenièvre would sound something like zhə-nye-VER
That is just a guess, so it may be incorrect.