Pacing is a very important, but often ignored concept in a lot of games. Many games artificially pad out their runtime with nebulous "content", just for the sake of saying that their game is >X hours long, instead of <X hours long, without necessarily paying much attention to whether or not that stuff is fun or engaging.
And like many games before it, Gears Tactics has run afoul of this as well.
In Gears Tactics, the side-missions are the issue. In this game, these side-missions manifest as combat missions, with recycled objectives, on recycled maps. They are useful for leveling up your troops and collecting gear, but they do not themselves advance the main story, or present any side narratives. What is so bad about these? They are mandatory progress gates. After competing certain story missions, you are forced to complete one, two, or three side-missions (depending on overall story progress) before the next story mission becomes available. These were certainly helpful during the early game, but I feel like I am powerful enough already, I really don't need any extra gear or levels, and now I need to complete three more side missions before I can begin the final level.
Gears Tactics is a fun game, and its combat is definitely enjoyable, but the game basically has no other mechanics to rely on or space things out, and it isn't incredibly deep to boot. Its worn thin, and needing to do more missions, for mission's sake, is a pretty exhausting finale, to what has otherwise been a pretty enjoyable TRPG.