If the standard is tank armor then no... 20mm isn't enough... Apache Helicopters, Warthogs, etc. are equipped with 30mm in order to be able to take on tanks and even then it takes sustained firing.
While keep in mind the Guyver armor doesn't have the same vulnerabilities as man made armor, one of the key factors of effective anti-tank rounds are the incendiary effect but Guyvers are all but incendiary proof and kinetic force that can shatter steel is the kind of kinetic energy that you get from tank range cannon shells.
20mm is effective against lighter armored targets but it's not the weapon you use to go after tanks...
Consider, the concept of the Guyver is that it was originally some sort of universal spacesuit/armor for the creators... In space you have to deal with things like micro-meteorites, which can hit you with the force of a grenade... The Guyver can clearly withstand a lot more force than a grenade... The Guyver can punch hard enough to penetrate any regular Zoanoid armor and with some effort they can overcome the armor of some hypers...
In the Manga, G1 managed to break through Zerebubuth's back armor with maximum force, feet, first, hit... Despite it being impact absorbing enough to withstand the normal Guyver punch... Logically, the Guyver is tough enough to withstand its own blows... and there's a reason why even Chronos relies on Enzyme based acid to dish out any real damage to the Guyver.
There's also the less official example of the OAV, in which Chronos literally fired artillery at G1 and it had no effect...
So while I agree that the Guyver can be damaged by conventional weapons that it still has to be high range weapons above anything we would us on anything short of a heavy tank...
Something like that scene in the movie Battleship where they tricked the alien to stand on deck just as they fired the cannon is the ideal situation to do serious harm to a Guyver...
Preferably point blank to the head ;-p
Followed immediately by a MOAB to keep the body from going out of control...