Making my comment into an answer:
Was there a gap of knowledge or slow-down of progress in math as a whole between Ancient Greeks and the 17th century?
The answer is probably no.
Islamic Medieval World and Medieval India contributed greatly to math during the Middle Ages. The fields of algebra, polynomial equations, cubic roots, modern numerals, logic, negative numbers, trigonometric functions and much more came from that period. These contributions are not minor and contributed continuously to the development of math. Islamic sources in particular translated and preserved a lot of Ancient Greek and Indian sources, and Islamic sources were later used by Renaissance Europeans like Fibonacci.
Wikipedia's history of mathematics also cites:
Other achievements of Muslim mathematicians during this period include the addition of the decimal point notation to the Arabic numerals, the discovery of all the modern trigonometric functions besides the sine, al-Kindi's introduction of cryptanalysis and frequency analysis, the development of analytic geometry by Ibn al-Haytham, the beginning of algebraic geometry by Omar Khayyam and the development of an algebraic notation by al-Qalasādī.
As for well known theorems of this time you have: the law of cosines (a.k.a. Al-Kashi's theorem), Chinese remainder theorem, Merton mean speed theorem, Wilson's theorem, Thabit's rule, divergence of the harmonic series, and the binomial theorem.
But even if we forget the Middle Ages as some "Dark Ages" (a term now considered as misnomer) and focus on Europe, we still have the whole Renaissance. People like Tartaglia and Cardano helped develop the solution to the quartic equations. Bombelli developed the algebra of imaginary numbers.
Many theses could be written on why the developments between Ancient Greeks and 17th century math seem "slow" or minor. I propose three conjectures:
- Lack of proper sources and attribution: sources foreign to Europe were not translated or properly attributed. This makes a lot of contributions that have no authors attached to them (even if we have words like algebra that come from Arabic or algorithm which comes from Al-Khwarizmi).
- Utility: Some mathematical content is preserved and transferred to the next generation better than others. Most of the time if there is some application (engineering, physics or astronomy) there was a higher chance for the content to survive. Kerala School might have invented calculus, but clearly their results were never largely popularized (maybe even never left Kerala).
- Transition of power: As Europe declined in the Middle Ages and the Medieval Islamic World became an important center of knowledge and power, clearly there was a lot of friction. Text had to be translated from Greek and Latin (and Sanskrit from India) to Arabic - and when Europe "reemerged", back from Arabic to Latin. Religious conflicts and other power struggles also did not help. Imagine how hard it was to introduce Hindu decimal numerals from the Indian subcontinent into West Asia and into Europe without the Internet, printing, long distance sailing and so on. Nevertheless, it made the trip because it was an important breakthrough.
However for every reason that I can find for this possible setback in progress in math, I can find a ton of contributions from those times that make the setback negligible. How much importance we give to these developments over some more recent ones is more a matter of opinion or perspective - though this is not the forum for opinion-based discussions.
The Dark Ages is a myth. For a light read, check for example The Light Ages: The Surprising Story of Medieval Science by Seb Falk (Time excerpt), and The West: A New History in Fourteen Lives by Naoíse Mac Sweeney (Smithsonian excerpt).
MathSE recommends A Short Account of the History of Mathematics by W. W. Rouse Ball for the history of math in general. It divides all math in three great periods: Ancient Greeks, Middle Ages and Renaissance, and Modern (beginning with Descartes). Chapters cover math from the last period of the Roman Empire, Middle Ages math (both in Islamic and Europe) and Renaissance math.
I end by adding this good quote I found in MacTutor History of Mathematics Archive article on "Arabic mathematics":
There is a widely held view that, after a brilliant period for mathematics when the Greeks laid the foundations for modern mathematics, there was a period of stagnation before the Europeans took over where the Greeks left off at the beginning of the sixteenth century. The common perception of the period of 1000 years or so between the ancient Greeks and the European Renaissance is that little happened in the world of mathematics except that some Arabic translations of Greek texts were made which preserved the Greek learning so that it was available to the Europeans at the beginning of the sixteenth century.
That such views should be generally held is of no surprise. Many leading historians of mathematics have contributed to the perception by either omitting any mention of Arabic/Islamic mathematics in the historical development of the subject or with statements such as that made by Duhem in 3:-
... Arabic science only reproduced the teachings received from Greek science.
Certainly many of the ideas which were previously thought to have been brilliant new conceptions due to European mathematicians of the sixteenth, seventeenth and eighteenth centuries are now known to have been developed by Arabic/Islamic mathematicians around four centuries earlier. In many respects the mathematics studied today is far closer in style to that of the Arabic/Islamic contribution than to that of the Greeks.