knapp
scarce
Available only in a small, barely sufficient amount.
Die Zeit wird knapp, wir müssen uns beeilen.
Time is getting scarce; we need to hurry.

scarce / just under / close
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scarce
Available only in a small, barely sufficient amount.
Die Zeit wird knapp, wir müssen uns beeilen.
Time is getting scarce; we need to hurry.

In the meaning 'barely sufficient', 'knapp' (scarce) is very often used with abstract resources such as 'Zeit' (time), 'Geld' (money), or 'Platz' (space). It usually appears with the verbs 'sein' (to be) or 'werden' (to become/get).
Wir müssen uns beeilen, die Zeit wird langsam knapp.
We need to hurry; time is slowly getting scarce.
When 'knapp' (just under) stands directly before a number or quantity, it means 'almost' or 'a little less than'. In this function, it is an adverb and is not declined.
Die Reise kostet knapp 500 Euro.
The trip costs just under 500 euros.
When referring to results in sports or competitions, 'knapp' (close) describes a very small difference. Typical combinations are 'ein knapper Sieg' (a close victory), 'eine knappe Niederlage' (a narrow defeat), or 'ein knappes Ergebnis' (a close result).
Die Heimmannschaft verlor das Spiel nur durch eine knappe Entscheidung.
The home team lost the game only because of a close decision.
In connection with communication, 'knapp' (brief) means 'short and concise'. People speak of 'eine knappe Antwort' (a brief answer), 'eine knappe Erklärung' (a brief explanation), or 'eine knappe Zusammenfassung' (a brief summary).
Er war in Eile und gab mir nur eine knappe Beschreibung des Problems.
He was in a hurry and gave me only a brief description of the problem.
The meaning 'too tight' or 'too short' is used almost exclusively for clothing and fabrics. It describes that a piece of clothing no longer fits properly.
Nach dem Urlaub war ihm seine Lieblingshose zu knapp.
After the vacation, his favorite pants were too tight for him.
good
'gut' and 'knapp' can both come before numbers, amounts, or time expressions. In that case, however, they point in opposite directions. 'Gut eine Stunde' means a little more than an hour. 'Knapp eine Stunde' means a little less than an hour. That is exactly why learners can easily confuse the words: both make the number approximate, but one rounds upward and the other downward. A helpful rule of thumb is: 'gut' is a little more, 'knapp' is a little less.

Bis zum Beginn des Films bleibt uns knapp eine halbe Stunde.
We have just under half an hour until the movie starts.
Für den Weg zum See brauchen wir gut zwei Stunden.
It takes us a good two hours to get to the lake.