Also what decides on a long or short cage RD?
Long answer
The chain needs to be long enough to go around the big chainring and biggest sprocket, and the cage is needed to take in the slack in the chain that is introduced when using smaller sprockets and/or ring. The bigger difference between the two extremes, the longer the cage needed.
The amount of slack that a derailleur can cope with is called its capacity. It's the difference between (small chaingring + smallest sprocket) and (big chainring + big sprocket). So if you have 50/34 up front and 11-28 in the rear you need a capacity of (50+28) - (34+11) = 33
The R7000 GS (long cage) has a capacity of 39 so it can cope with 50/34 and 11-34
The R7000 SS (short cage) has a capacity of 35 so it can cope with 50/34 and 11-30 but not 32 or 34.
Specs:
https://bike.shimano.com/en-EU/product/component/shimano105-r7000/RD-R7000-GS.html
https://bike.shimano.com/en-EU/product/component/shimano105-r7000/RD-R7000-SS.html
Someone will no doubt be along to say "didn't you know you shouldn't ride in big-big blah blah blah"
Someone else will probably be along to say that they've used a short cage mech with some unfeasible combination and it worked. Yes, the manufacturer's specs can be exceeded.
Edit to add:
Note that the derailleurs are also rated with min and max low sprocket, so if you go for a GS derailleur your biggest sprocket should be between 30 and 34. You shouldn't run a cassette with biggest sprocket of 28 with this if you want to stay within the manufacturers limits. It would probably work though. Similarly if you have an SS you shouldn't run a cassette with a biggest sprocket of 30 or more regardless of capacity considerations. But the manufacturers limits are probably a bit conservative.
Short answer: Unless you are a super fast powerful climber, or you live in an exceptionally flat area: Go for the GS.