I need to develop GIS-related content in Java environment. I want to know whether the GDAL library and GeoTools are functionally the same. Can you use GeoTools directly to implement most of the functionality of GDAL?
TL;DR Sort of, probably, it depends.
GDAL provides a lot of import and export formats and a few specific tools to manipulate raster data. I'm being a little old fashioned here and assuming that when you say GDAL you only mean rasters (if you also mean OGR and vectors then it mostly still applies).
Meanwhile, GeoTools provides a pluggable data import and export system to allow you to use and convert many types of raster (and vector) data and a full ecosystem of processes that can be applied to that data. It even includes Jiffle to allow you to allow your users to write scripts to process them.
Finally, GeoTools allows you to style and display (or print) your data using OGC standard SLD styling (or CSS and YAML variants).
Obviously, as a GeoTools developer I'm probably biased but in you are doing geography in Java then GeoTools is a great tool to do it.