Yes there were gun ships and makos and they were blown up all the same trying to get to the beam. Soldiers of any kind are not kittens that can be distracted from things because they move really fast by them. Particularly the disposable way Reaper's use troops. The gun ships would only retain attention as long as they were around the troops. Which means they would also be targets to be shot at and killed. Thessia already showed the effect of trying to use a Gun ship to protect ground troops for extended periods. It works short term then the Reapers target it and blow it out of the sky. Tunchunka was close to jets then gun ships which are more like modern day helicopters. And they even admit that they had limited attacks only 2 maybe 3 passes before they were rendered useless. Part of the tension of the run up to it. Because they had to get there now before the Turian ships use up all their missiles and are rendered useless.
You know why those gunships and makos got blown up?
Because they were charging headlong at Harbinger not even trying to evade. Seriously, what was the point of them even being there?
Now what was the strategy on Tuchanka? Oh, yeah, gunships and vehicles distracting the Reaper (who knows they had to get t the Shroud) So Shepard and Mordin could slip past itAnd the only reason it failed was the tomkahs got held up. But even two or three passes would have been a lot more coer than the infantry got on Earth.
Harbinger isn't an idiot it knows the beam is the point they are trying to make. It won't leave the beam because some people start to shoot spit balls at it. The only reason Harbinger even shows up is specifically because they were able to break though the Reaper line and almost made it to the beam. It's arrival was at the right time to protect the beam by blowing the **** out of everyone near it. You wouldn't be able to lure it away though a few rounds of a gun or a Makko cannon. Those are like butterfly kisses to them.
We don't need Harbinger to leave the beam. We need a few soldier to reach the beam alive. What's Harbinger going to do then? Chase them? Inflicting any damage to Harbinger at all would simply be a bonus
Reapers are guarding the entire area around the beam. Seriously you can't seem to follow basic logic and it is painful talking to someone who lacks even basic understanding of military strategies. The Reapers are spread out so the Allies pick a single attack route and hit it hard and fast. Break though the Reaper line and get into the beam before they can move troops to reinforce the area they are being attacked in. Once they reached the beam that would create a choke point for the Allies to hold off against any Reaper forces trying to follow them into the beam.
Believe me, I know exactly how you feel 
Wait, If the beam is surrounded by Reaper forces to the point where only one spot in the reaper line can be punched through, where is this hypothetical "choke point" where the allies can hold off Reaper forces? Won't they find themselves cut off and surrounded?
Aren't the allies fighting their way through a Reaper choke point here, by your logic? Which would be, frankly suicide.
Seriously that was the entire point the Greeks picked the area to fight during the Battle of Thermopylae. The geographical location created a natural choke point. So even though the Persian Army massively out numbered them the narrow fighting area limited the number of troops they could use against the Greeks. Evening the fight a bit. 7,000 men held off 150,000 for 7 days because of that natural choke point.
Didn't the Greeks lose Thermopylae because the Persians found an alternate path that allowed them to surround the Greeks? 