Note: Gunbreaker rotation will do different things depending on which junction you have activated, this will mean that you can either get SHIELDS on tank junction, HEALS on healer junction, or damage on dps junction.
Note that I wrote damage in lowercase, because this is the worst junction.
Healer junction gives you the most heal ability, this allows you to also cast a heal over time on party members or yourself AND heal every time you pull off your continuation (will be explained later)
Tank junction gives you more shields than the heal ability HOWEVER you can only cast heals on yourself AND shields are temporary vs health. These shield abilities also tend to be a one and done thing vs a heal over time like healer junction
dps junction just increases your damage, that's about it, you do sub dps damage but still actually hit somewhat decently hard, normally you do no damage but as a dps junction you're just as squishy as a dps (even though you're a tank) and do less dps than dps. In 1v1 this is the worst, maybe useful while chasing
Your healer junction will in general give you more sustain for a longer term fight, the junction cast has a longer duration and in general all the skills heal your health. This means your sustain is not temporary unlike tank junction. Tank junction however if you can predict skills and time everything is more powerful as it's potency is stronger... Problem is with tank junction you only have the default healing skill if you lose health. If I'm being bursted down I would prefer tank junction, however if I have enemies with alot of sustained dps over a longer period of time I'm more comfortable with healer junction. Healer junction in general seems to be more newbie friendly as well since you don't need to know and predict opponent attack patterns for optimal efficiency (spam heals + pop heal over time prior to more risky chains or your lb)