There's a few options that exist currently to make these characters viable that I think the developers strongly need to consider. Nobody likes change but sometimes the status quo isn't satisfactory.
Option 1: just shrink them
This fix applies to both caustic and gibraltar. What could be a more simplistic solution to such a simple problem? No new assets would need to be created, just shrink preexisting models and hitboxes.
Option 2: just delete them
In all seriousness this seems like the only alternative if the developers don't want to properly fix them.
Option 1: It's an option, but a bad one I think since I like the variety of legend properties.
Caustic can be fixed by making it impossible to disable traps without them triggering first and decreasing time of ultimate cooldown. If this still is not enough then boosting Fortified perk to 15 or 20% is always an option.
Gibraltar can be fixed by making his ADS-shield bigger and not have a delay before popping up. He also needs double Fortified.
Option 2: Nah, we need more legends, not fewer, and lots of people have grown fond of these two.
That depends if you want them to be powerful towards early game or make them more balanced through the whole match. I vote for the latter