Yes. Linos can't get +4. Only +3.
To better emphasize: the max augment Linos can get is
+2 for a total of
+3.
Gjallarhorn is the cheapest of the BRD RMEAs which, alas, are pretty much all mandatory if you want to do "serious" content on BRD.
As such yes, Gjallarhorn is totally worth it.
At the same time though, Gjallarhorn used to be our only source of song+4, and only a couple of songs here and there could get additional +1.
Now the situation is different. You get up to +3 from the neck, and then some songs can get up to +2 from other slots.
This means that considering the maximum possible value you can reach for each song, Gjallarhorn represents in 2019 a smaller % than it used to be (where it was like, what, over 90%?).
From this perspective I think it could be considered "acceptable" to have a Linos with a total of Song +3, you would lose "only" +1 song, which is not ideal but frankly it's viable for the majority of content.
Talking briefly about the other REMAs:
1) Marsyas => pretty much a must in nowadays' meta
2) Carnwenhan => mostly a QoL that optimizes how long you can make your good stuff last. It hardly makes a difference for short fights. It used to be a big deal on fights like Vinipata and Albumen but I guess no more.
3) Daurdabla => for mules used mostly for zergs (where you use CC and SV) it's probably not a big deal and you can use Terpander just fine in its place. For main characters I'd say having an additional song is pretty much a big deal. A VERY big deal, especially for lowmen content where you don't get the luxury of BRD prebuff or BRD swapping.
All of these described from the perspective of Buffing. There's more to it when you consider the Debuffing aspect but I left that out on purpose.