I am doing up an old Top Hat 25 and part of the boat mainly the top deck has large areas of fine cracking in the gel coating (which is dull). These are extremely fine and do not appear deep. What is the best repair method? I am planning to replace windows and paint in an area that is affected by the this fine cracking. Repair solution required?

There probably will be bits of information in this prior thread of use but your crazing maybe looks a bit different.

www.seabreeze.com.au/forums/Sailing/General/Deck-repair-advice

In the 2nd photo it is clear that the crazing is in the original smooth deck / cockpit / cabin area gelcoat but for the first photo is that the original nonskid pattern gelcoat or a nonskid paint applied over the original smooth deck gelcoat, or other?

For the 2nd photo the root cause is probably just age and uv deterioration as there doesn't seem to be crazing at radii of the moulding transition corners which would be load/stress related due to too thin a laminate. For the areas like in the 2nd photo would suggest the only cure is a good sanding (battery powered orbital sander) then acetone clean then a thinned coat of epoxy primer carefully brushed on. It might need more sanding after that has cured, and another coat to fill it all properly. Then more fine sanding and 2 coats of polyurethane suggest satin gloss and it should come up very well with good quality brushes and application. Can recommend Norglass products but they will need thinning with the correct thinner.

For the 1st photo area more info needed as above.

The only permanent cure I ever found was to open up the spiderweb with a dremel and to fill , preferably with epoxy but at a pinch , especially if you want to avoid polyurethane painting the whole deck, you can get away with colour matched flow
coat thickened with micro balloons. This is a bit easier to sand smooth than epoxy filler, and doesn't go yellow in UV like epoxy.
The reason for opening the cracks with a dremel is that when examined under magnification , a gel coat crack has near parallel sides and is inevitably contaminated, so your resin / filler will only penetrate a superficial distance inside the fissure and you are going to find it difficult to get a decent lasting bond. By opening up the spider web ( typically takes only about 1 minute per spider entire web ) you will actually save time in the overall job because it will be much easier and quicker to get the filler into the crack. And it will be more likely to stay there .
Finally, whatever you use has to be thickened and forced into the crack. Also, as I reckon most people have experienced, Just painting the spiderweb without filling it won't work. It won't work because the paint won't go in, it will either just form a lip at the edges of the crack ( usual) or just cover it over if you persevere when repainting it. Maybe others have had different experiences but that has been mine.
Cheers, Keith.