As I am in a window replacement job just now, I can tell you that when removed, mine showed liberal use of either 3M5200 or Sika 291 (I can't tell them apart) which, at 38 years in, is still in "as new" condition.
Unfortunately, without taking the window assembly apart, it is impossible to tell how it was originally put together. If any GB owners have done so, perhaps you will get some wisdom from them.
My fixed window frames are well designed, the slider frames, not so much.
Fixed: the frames are fitted from the inside, with screws from the outside FG of the house going into the teak frame, which is also 291d onto the inside of the FG. next the glass goes in, then the outer frame is 291 and screws onto the outside.
Sliding: top 3 sides as for the fixed units. Bottom, designed to fail, with the stiffening plywood that makes up a structural part of the house sides coming right up to the bottom of the opening in the FG. ( it looks as though the location of the window opening was not designed into the mould, but was cut out later, so through the FG and plywood), then a 1/4 x 1 1/4 strip of teak was set onto the cut edge, on which the slide track was fastened with 5200 or 291. Any fracture in the integrity of the joinery below the track would allow water to enter and damage the interior finishes, ultimately putting the whole area at risk of rot.
I know that most (all?) GB opening windows have a drain pipe protruding below the rear corner of the window, so perhaps some designer has eliminated the above concern in your boat. Again, check to see if there is any GB wisdom on this issue.
If you are satisfied in the integrity of your installation, get down to clean 291 with a knife or chisel, then you can add new 291 on top, secure in the knowledge that wherever it touches clean material, it sticks for life.
