I use Signal without gapps and do not have extreme battery drain. I always get real-time notifications too - no late or missing deliveries. This is both on a galaxy s9 which went from stock ROM to deleted Gapps with no battery impact, and s 1+ 7 Pro with no Gapps from the very beginning (I get the expected excellent battery life).

I suspected that Signal basically took Over Gapps's role since it's the only app I use with notifications. Google keeps open a single optimized connection and apps go through it instead of having 10 open connections. After deleting Gapps, Signal is the only app keeping open a connection so it basically has the same impact.

But then I installed WhatsApp and Signal+WA both work perfectly without Gapps and with no gigantic battery drain. So I have no idea what black magic is making this work.

I just looked up my battery stats; since the last full charge 32 hours ago Signal has used 1% of the power. I got a few notifications and used the app for a few minutes in this time. WhatsApp's power usage is too low to show up in the stats.