Praise be to Allah.
The one who has to do ghusl for janabah must make the water reach every part of the body. Please see the answer to question no. 103738 .
If there are any patches, dressings and the like on his body, he must remove them when doing ghusl, if there is no fear of harm. If there is fear of harm, he may wipe over them.
Ibn Qudamah (may Allah have mercy on him) said in al-Mughni (1/173): Al-Qadi said, regarding dressings on wounds: If no harm will result from taking it off, then he should take it off and wash the healthy part, and do tayammum for the wound, and wipe over the site of the wound. If taking it off will lead to harm, then it comes under the same ruling as plaster casts, and he should wipe over it. End quote.
Undoubtedly these patches prevent the water from reaching the skin, so if it is not essential to wear them, ghusl for janabah will not be valid unless one removes them, because of the obligation to make the water reach all parts of the body.
Based on that, whoever wears this patch and wants to do ghusl for janabah must remove it, and it is not permissible to wipe over it. It cannot be compared to a plaster cast, because wearing it is not essential, and the traditional ways of testing blood sugar mean that there is no need for this patch, as those ways are easy.
We put this question to our Shaykh, ‘Abd ar-Rahman al-Barrak (may Allah preserve him) and he replied:
Patches like these should be removed when doing ghusl for janabah, because they are not like plaster casts and it is not essential to wear them. Doing finger pricks is something easy.
And Allah knows best.