what you want to get is polyurethane sealant , its comes under various makes but a commonly used type in car repairing is SIKAFLEX 221 .
it will stick or seal just about anything and will seal youre gutter easily .
costs around 7 pounds per tube but you need the longer mastic gun to carry the poly sealant tubes, ie a 300mm long gun .
when applying it you just lay a bead of sealant along down in the gutter and then get youre finger and a cup of either white spirit or synthetic thinners and after wetting finger you just drag it along to form a nice uniform shaped fillet . with wetting the finger youll make the surface of sealant slippery and easy to manipulate into shape .
you can put on vinyl gloves if you want proper protection from the chemicals but most of use just apply the sealant and then clean our hand straight afterwards .
you can buy the poly sealant in various colours but usually its mostly supplied in white or black .
you can paint over it .
just allow about 8 hours for the sealant to go off , in normal temps . longer in cold .
if you use silicone sealant you wont be able to paint over it, paint wont stick to it .