Obtaining data table headers from GEO using GEOquery
The short answer is that if you are seeing information in a GEO sample, then it is the sample that you need to download and access using the appropriate method. In this case, that method is Columns():

library(GEOquery)
gsm &lt;- getGEO("GSM2644971")
Columns(gsm)

          Column      Description
1         ID_REF                                                            
2         VALUE       Normalized (provided the normalization method) Average Beta
3         Detection   Pval


The longer answer: you will really benefit from sitting down with the GEOquery documentation and trying to understand the objects that it uses and their methods. It is not something you can work out by trial and error for every case-by-case basis. The tutorial is a really good starting point (you will find Columns() in there, for example).