It seems that "gum dam" both in that phrasing and as "rubber dam retaining devices which use cervically directed forces to cervically retract and retain a rubber dam below the gum line in the mouth of a patient during dental treatment" are definitely terms used in this context.

Frome the same site, "The invention has implications for general field isolation in dentistry, conventional isolation, and a hybrid type of isolation which involves both conventional isolation and general field isolation simultaneously." So, field isolation would also seem to be correct.

However, I've put a low confidence level due to my general lack of knowledge in this area and also because I ended up with rather a literal transaltion - which can surely be improved upon.

More context would help, too!

http://www.freepatentsonline.com/20040209224.html