alien3d just to further explain gear re-rolls

On every character gears page you'll get to see whether they're using physical or energy damage.
Exception : Phil Coulson & Ghost Rider they have skills that uses Energy & Physical damage, best to roll All Attack to get the most out of this 2
Exception2 : Hulkbuster, his gears will show All Attack, but in actuality all his damage comes from his Physical attack stat.

Select the stat that you wish to modify.
Press the change button to modify. Result of modification is random based on the range of values stated on the right hand side dialog box as shown
For the first 2 option. It's better to roll for %by LV based stat as it gives more that rolling for flat value
Take for example as this picture show. The maximum flat value that you can roll is Physical or Energy attack + 29
Basing on the picture above as example a Energy Attack By Lv +52% in translation to a flat value would be 52 X your hero level / 100
So lets say your Floki is level 60, using the example above, it'll translate to about Energy attack +31 which gives slightly more than getting a flat value of +29.
From slot 2 onwards it's better to change to a flat value .

Alternatively you can press your skill description to see what type of damage that the skill requires.
Kindly note that Floki 6* skill does not benefit from cold gear (it has an Energy label next to it)
Hope this clarifies a thing or two. You can always check
hitz frddst Davidtcf heroes to have a rough idea what stat to roll for the other 3 gears