"Kumpulkan mereka di sini."

Translation: Gather them here.

[Keith_APP]

This mereka applies to things too?

[DiegoJaviUnlam]

Halo teman! The plural for things would be "itu" or "ini". Example:

Those apples are delicious. Gather them here. / Apel-apel itu enak. Kumpulkan itu di sini.

Breaking it down:
Those (itu) apples (apel-apel / apel) are (pause / zero copula) delicious (enak)
Gather (kumpulkan) them (itu / ini) di sini (here)

Even, I have seen some mistakes before in the course for Indonesian speakers, those have been fixed. Also, maybe some native speakers could mix "itu" with "mereka" in colloquial speech.

I hope it helps.

Selamat belajar! :)

[WayangOrang]

This mereka applies to things too?

'mereka' applies to humans ('orang').

https://kbbi.kemdikbud.go.id/entri/mereka

orang ketiga jamak (dia dengan yang lain); orang-orang yang dibicarakan:
Toto bertemu dengan Amir di jalan, lalu -- pergi bersama-sama ke sekolah

3rd person plural (he/she with others); people that you talk about

[Mal205614]

"gather them together here" should be accepted too? as kumpul means gather together