Potential counters to guncom on astro:
-Air snipe (lots of t1 bombers, or rush to t2 and get t2 bombers)
-T2 upgrade on com followed by 2 T2 pd (if against just 1 guncom); you need to rush the T2 upgrade and get the PD behind some of your mexes to give you enough time to get 2 T2 pd up before the guncom gets in range
-Aeon guncom (if they're not aeon) since it allows you to kite them
-If they're at mid upgrading with no T1 land or another com and aren't aeon, then if you're quick you could try sending 3 T1 arti to attack them while upgrading, meaning they either take some damage or are forced to cancel
-Guncom+teammate (if vs 1 guncom)
Guncom works best where you can get it early enough that they can't stop you with T2 pd, and if you're against just 1 enemy commander. However it costs significantly more power than T2, so your eco will suffer if you get it meaning you need to get a decent amount of kills for it to be worthwhile.