Gestalt therapy and existential therapy are different modalities, but they both come under the humanistic umbrella, along with person-centred, transactional analysis, focusing-oriented... and many others.

Humanistic therapies have in common the central premise that human beings are inherently capable of change and growth and that the therapist's role is to provide an environment conducive to that.

Gestalt therapy is very focused in the present moment, and places an emphasis on personal responsibility. My understanding is that some therapists can be quite confrontational in pursuing that focus.

I don't know a lot about what existential therapy is like in practice, but, as is clear from the name, it encourages a focus on existential issues.

I wouldn't see a T who wasn't humanistic, but my experiences have certainly varied. I think T1 would claim that he was practising gestalt therapy with me, at least in part, but I don't know if I agree. He liked to think he was like Fritz Perls (gesta therapy founder, also a nasty misogynist). None of my Ts have claimed to be existential though I think there has sometimes been an existential element to our work, and current T definitely has an interest.