The Glue Gunner is a really good tower for slowing bloons down. Additional upgrades can make it pop bloons!
It is generally best placed near the start of a track or around a strong tower. Late into the game though, you might actually want to place a glue gunner in the middle of the track.
It becomes near-useless in the mid-game rounds when there are very large amounts of bloons, until the Glue Splatter or Glue Hose upgrade is purchased. One Glue Gunner is not as effective in the middle rounds, so it is recommended that you have a couple of these all upgraded to Glue Splatter at least.
A deadly strategy is to put a glue gunner near the exit (the moabs will be popped) and upgrade it to 2-3. Then put a 3-2 ice tower next to it, and virtually no bloons (except moabs) can get through. This is for round 60+. It works very well with the glue gunner upgraded to 3-2 as well. You might think it does not have time to glue all bloons but it really does, since the ice tower which slows down the bloons quite a lot, gives it more than enough time.
Sell your Glue Gunner once you have passed round 85 in Bloons TD 5, because the only bloons that'll come are M.O.A.B.s, B.F.B.s, Z.O.M.G.s and Ceramic Bloons, but by then you should have towers strong enough to pop Ceramic Bloons with ease.
Getting Bloon Liquifier or Bloon Dissolver instead of a Super Monkey in the earlier rounds can be considered.