Learn Hardcore Croatian: Mikrofon se gasi usred videopoziva, pa se jedva čujemo. - The microphone keeps turning off in the middle of the video call, so we can barely hear each other.

Here se is not truly reflexive (the microphone isn’t “turning itself off on purpose”). It’s a very common Croatian use of se to express something happening “by itself / automatically / unintentionally” (a so‑called middle or inchoative use).

Mikrofon se gasi ≈ the microphone keeps switching off / is cutting out (on its own). Without se, Mikrofon gasi… would normally mean “The microphone turns off (something else)” or would sound incomplete.

What’s the difference between gasiti and ugasiti here?

They’re an imperfective/perfective pair:

gasiti (se) = imperfective: an ongoing/repeated process, “to be switching off / to keep cutting out”
ugasiti (se) = perfective: a single completed event, “to switch off (once), to go off” So:
Mikrofon se gasi… suggests it happens repeatedly or is in the process.
Mikrofon se ugasi… suggests it switches off once (at a particular moment).

Why is it gasi (present tense) if this is about something happening during a call?