The use of what appears to be silicon (but which could be something like a polyurethane) is a common practice by GSO. GSO has been using this substance on its large 16" dobs for at least a couple of years. It is on my 16" dob between the mirror carrier and the glass, three blobs spaced 120 degrees apart (and equi-distant between the clips. It has been my view that the purpose is to further support the heavy thick mirror and prevent shifting in the carrier or pinching against one of the clips, which can be a problem during transport or at low angles of operation. It does not affect my mirror performance in anyway that I can see as the substance is compressible and will yield with any expansion or contraction of the carrier or the glass. My collimation stays perfect through changes in temperature experienced at Bretti (where its not unusual to have hot days followed by cold nights). I have not trianglular starts. This substance has been noticed by the 16" dob owners on Cloudy Nights and in reviews without any negative performance credited to its presence.
Some people have cut it out (using a wire saw) mostly so that they can remove the mirror for cleaning, but most don't bother as there is no link with an negative effects. I suspect GSO would have tested that before they used it.
It is a good idea to ask GSO why its there, something I have not done. I'd suggest not removing it until you get a reason from GSO as to why its there. You can email them here:
service@gsotelescope.com