library(Biobase)
library(GEOquery)
library(simpleaffy)

gds4758 <- getGEO('GDS4758',"GDS4758.soft")
eset <- GDS2eSet(gds4758, do.log2=TRUE)
sampleNames(eset)
 [1] "GSM907858" "GSM907859" "GSM907860" "GSM907854" "GSM907855" "GSM907856"
 [7] "GSM907857" "GSM907825" "GSM907828" "GSM907832" "GSM907833" "GSM907834"
[13] "GSM907826" "GSM907827" "GSM907829" "GSM907830" "GSM907831" "GSM907792"
[19] "GSM907795" "GSM907801" "GSM907802" "GSM907804" "GSM907805" "GSM907806"
[25] "GSM907793" "GSM907794" "GSM907796" "GSM907797" "GSM907798" "GSM907799"
[31] "GSM907800" "GSM907803" "GSM907864" "GSM907865" "GSM907868" "GSM907869"
[37] "GSM907870" "GSM907861" "GSM907862" "GSM907863" "GSM907866" "GSM907867"
[43] "GSM907839" "GSM907840" "GSM907842" "GSM907843" "GSM907845" "GSM907846"
[49] "GSM907848" "GSM907851" "GSM907835" "GSM907836" "GSM907837" "GSM907838"
[55] "GSM907841" "GSM907844" "GSM907847" "GSM907849" "GSM907850" "GSM907852"
[61] "GSM907853" "GSM907807" "GSM907813" "GSM907814" "GSM907816" "GSM907818"
[67] "GSM907819" "GSM907820" "GSM907822" "GSM907823" "GSM907808" "GSM907809"
[73] "GSM907810" "GSM907811" "GSM907812" "GSM907815" "GSM907817" "GSM907821"
[79] "GSM907824"

geneNames(eset)
Error in (function (classes, fdef, mtable)  : 
  unable to find an inherited method for function ‘geneNames’ for signature ‘"ExpressionSet"’

Please help me with this error ! The dataset i used here is that of alzheimer disease.

There isn't a geneNames() function for ExpressionSets any longer. There is a featureNames() function, which will give you the manufacturer's ID for each reporter on the array. But I suspect that isn't what you want. If you want the HUGO gene symbol, then you need an annotation package for this array.

These appear to be Human Gene 1.0 ST arrays from Affymetrix, and the submitter says this about the processing:

"CEL files were imported into Affymetrix Expression Console Software, and CHP files were generated."

Which is like, helpful and stuff, so let's assume they summarized at the transcript (or core) level, so you want to annotate using the hugene10sttranscriptcluster.db package. See the vignette(s) for AnnotationDbi for more information about how to accomplish that.

You can check to see if the data are summarized at the core level by doing something like

all(featureNames(eset) %in% keys(hugene10sttranscriptcluster.db))

after installing and loading the hugene10sttranscriptcluster.db package, of course.