What do you mean "cut the rope in half"? If you mean to change the length, no problem, you will need to in order to get a proper fit... If you mean lengthwise, NO... you have the wrong size gasket, get something smaller...
In general the process to change gaskets is to start by getting rid of all the old gasket and cement - I try to chisel out as much as I can, and then use a dremel tool with a stone to get it down to bare metal in the gasket groove (I also hold a vacuum near the tool to suck up the mildly noxious dust...) Surface prep is important to doing a good job!
Fit the gasket to the groove by laying it into place, all the way around without stretching or squishing it, and trim to length plus an inch or so - use a sharp pair of scissors or a utility knife and try to get as clean a cut as possible, w/ no dangling threads. Try to put the joint where the original was, or in the middle of a run where it will be disturbed as little as possible (not real critical on a window, on a door I try to go for the hinge side) Take it out and put it aside after getting the length
Then put in a thin layer of refractory cement - I've never had much luck with those toothpaste tubes, they are worse than ketchup bottles... Instead I get a tub of cement, and without stirring, scoop a little out onto a throw away plate, and from there smear a uniform layer filling about 1/3 to 1/2 the groove w/ cement. (If you wash your hands with hot water quickly afterwards the cement will come off pretty easily) There will be a layer of liquid on top of the cement in the tub, leave it in the tub and try to mush the unused cement down to keep it under the liquid. Do NOT return any unused cement to the tub, throw it away... (This will keep the cement in the tub from hardenning on you) reseal the tub tightly when the job is done...
Then quickly lay the gasket into the groove and push it into the cement as evenly as you can, all the way around the groove - again don't stretch or squish the cord. When you get to the joint, trim cleanly to the exact length, and sort of mush the ends together to make as complete a seal as you can.
As soon as the gasket is in place, put the glass back on it and tighten it down, or if it's between other parts put them together and close them to flatten the gasket out and get a good seal. If it's something like a door gasket, I like to put a layer of waxed paper around the contact area to keep any cement squeezeout from sticking things together that shouldn't...
Gooserider