The same Gitlab offers: the ability to customize and configure the system the way you want it, and the ability to patch the system even when the vendor stops supporting it if you're intent to. It also adds a small factor of trust because you can see the server code; there's probably important, private files flowing through the relay servers, after all.

The rates for decent remote support software are terrible. For €150 per month, you can use three simultaneous connections across your entire organization, limited to a certain number of users who might possibly use the system. Teamviewer once offered lifetime licenses, but then they inserted ads for their subscription model into the lifetime customers to get even more money out of them.

[Not open source, but] Liteviewer has been the best of what I could find for running a tiny pseudo-MSP on the side. Despite their site looking like it was ported from Geocities, the app itself has been pretty solid for my basic uses, and ~30 machines are free, I believe. I'm not affiliated.

http://www.litemanager.com/produkts/LiteManagerFree/

These tools behave like trojans, allowing complete control over a machine. Why would anyone want to run something like that without being able to check for malicious code is beyond my comprehension.

The industry is notorious (lead by TeamViewer) for leveraging vendor lock-in to screw their customers as much as possible.