I would like to apologize before hand about the length of this post, but if you read it, it actually makes sense
0_0 I was watching the Bleach episode where Ichigo brought out Zangetsu for the first time, and it got me thinking, when Tessai first saw Ichigo's shikai form for Zangetsu, he was freaking out about Zangetsu. Which also leads me to another question, later in the arc when Ichigo uses Getsuga Tenshou against Renji, he flash back to Urahara telling him that knowing the name of the attack is increases the power, and when he says that, it kinda reminds me of how ichigo's initially powers up to use Getsuga Tenshou.
Which brings me to my point, I don't actually believe that Getsuga Tenshou is a zanpakutou based attack like we have been thinking. If you think about it, the basis of the attack is the compression of the users reiatsu and than by swinging Zangetsu, Ichigo is able to use that technique for offense and defense, and now we have seen that Isshin is also able to use Getsuga Tenshou and that there is even a final version of the attack. Now if Getsuga Tenshou was a normal zanpakutou attack like we see with Byakuya's Senbanzakura or Urahara's Benihime where they have attacks that are only seen with those zanpakutous. However, Getsuga Tensho has been seen by both Ichigo and Isshin. Because of this, it makes me think that Getsuga Tenshou is actually a attack that nearly anybody can use, very similar to kido spells. the catch is that someone would have to have a massive amounts of Reiatsu like Ichigo and Isshin have. I hypothesize that because of the amount of Reiatsu an attack like Getsuga Tenshou requires, the user would have to have a massive amount of reiatsu or they would quickly run out of power or would simply be unable to compress the necessary amount of reiatsu to be able to do much of anything. Another thing that makes sense with this theory is that Isshin used Getsuga Tenshou without his zanpakutou his shikai form, and since he has the ability to control his reiatsu better than Ichigo can, he might be able to keep his zanpakutou in its released form.
Now, I realize I answered my second question before my first question, only because I got carried away

Anyways, this question should be answered in a shorter manner. My belief as to why Tessai freaked out when he first say Zangetsu, its because Isshin has a similar zanapkutou shikai and potentially even bankai, though this theory doesn't really have any actual proof unlike my previous theory. Though it has been shown that Isshin and Urahara seem to know each other and even have the abilty to fight well together, leading me to believe that they are old allies, which also means that Tessai would also know Isshin and potentially his Zanpkutou, its shikai and even its bankai form. Because of tihs potential knowledge, when Tessai saw Zangetsu for the first time, he probably mistook it for Isshin's Zanpakutou and maybe even immediately knew how powerful it and Ichigo would be.
anyways there are my two theories about Getsuga Tenshou and Isshin's ability to use that attack and also teach the final form to Ichigo