How to get multiple multirows in a table?
I believe your problem is that you didn't put enough empty cells for the following lines. A multirow will allow content to occupy space, but doesn't actually create the cells that are under it. I rewrote your code aligning everything to illustrate that.
\begin{tabular}{l r | l || r r}
Vzorka & Výskyty & Algoritmus & Porovnania & Prístupy \\ \hline
\multirow{5}{*}{Gabriel} & \multirow{5}{*}{8} & NAIVE BINARY & 158 & 44 \\
& & NAIVE BINARY_LCP & 174 & 92 \\
& & NAIVE INTERPOLATION & 168 & 78 \\
& & LESS NAIVE BINARY & 98 & 26 \\
& & LESS NAIVE BINARY_LCP & 124 & 58 \\ \hline
\end{tabular}