I agree with the above. I would use something to call out the keyword. Either Bold or Small Caps or something.
Using words like Bolstered is fine, especially if you want to set a precedence for the term. You should just reduce the need for skimming ahead to look at keywords and include it in the statement.
I.E. "Once a target is either affected by this spell or succeeds at their save they are bolstered against its effect and cannot be affected by it for the next 24 hours."
You would then also include the term Bolstered in the keyword section and clarify what exactly it's intended to mean when used while describing an ability or spell.
I am afraid that Gloom's wording could be read as both bolstered against the effect and cannot be affected by it for the next 24 hours. Then the player would check the rulebook to see what "bolstered" mean and would be disappointed that it did nothing extra, because "bolstered" sounds like a benefit in contrast to the disadvantage in the second part.
Which is actually a fun idea. Hey, your character is bolstered against further healing by Battle Medic because the Battle Medic wound tending causes the patient to heal 1 hit point per minute, which would not stack with further use of the Battle Medic feat.