Quicklook works on my Mac (High Sierra) without Gapplin, but it generates small preview with white borders, which is not preferable.
If I install Gapplin (1.6.1), no matter it's the default app or not, it succeeds only once when I open Finder and select an .SVG file the first time. After that, quick look gives me a blank square if I select another file or switch back to the first figure.
I tried to execute
qlmanage -r; qlmanage -r cache, which doesn't solve the problem.
qlmanage -p test.svg does open the quicklook window, yet with error message:
qlmanage[17643:30371665] *** CFMessagePort: bootstrap_register(): failed 1100 (0x44c) 'Permission denied', port = 0x1361f, name = 'com.apple.tsm.portname'
See /usr/include/servers/bootstrap_defs.h for the error codes.
Gapplin always works to open a .SVG file.