How to extract gene expression tables from this GEO dataset?
Assuming that you have downloaded processed file, according to your link, it is either plain text (compressed) or GTF. If you would have plain text you probably would have posted a snapshot or sample of it, so I assume you have the GTF. According to format description for GTF-files: 


Format consists of one line per feature, each containing 9 columns of data, plus optional track definition lines. 
The 9 columns (or fields) are tab-separated and all fields contain a value (missing/NA-values are usually denoted with "."). 
Fields are: 


seqname - name of the chromosome or scaffold; chromosome names can be given with or without the 'chr' prefix. Important note: the seqname must be one used within Ensembl, i.e. a standard chromosome name or an Ensembl identifier such as a scaffold ID, without any additional content such as species or assembly. See the example GFF output below.
source - name of the program that generated this feature, or the data source (database or project name)
feature - feature type name, e.g. Gene, Variation, Similarity
start - Start position of the feature, with sequence numbering starting at 1.
end - End position of the feature, with sequence numbering starting at 1.
score - A floating point value.
strand - defined as + (forward) or - (reverse).
frame - One of '0', '1' or '2'. '0' indicates that the first base of the feature is the first base of a codon, '1' that the second base is the first base of a codon, and so on..
attribute - A semicolon-separated list of tag-value pairs, providing additional information about each feature.



You can find more information on it here and here. 

IF you don´t have the file and just have the query output, you probably will find the answer here, where extraction of expression data has been discussed in more detail.