If you have your sequences in a FASTA file, you can use awk to calculate GC content. The following will calculate GC content over all input sequences, and ignore ambiguous bases (like N):

$ awk ' \
BEGIN { \
    FS=""; \
    cg=0; \
    t=0; \
} \
{ \
    if ($1 != ">") { \
        for (i = 1; i <= NF; i++) { \
            if ($i ~ /[ACTGactg]/) { \
                t++;
            } \
            if ($i ~ /[CGcg]/) { \
                cg++;
            } \
        } \
    } \
} \
END { \
    print cg"\t"t"\t"(cg/t); \
}' foo.fa

For example:

$ more foo.fa
>ctcf
CCGCGNGGNGGCAG

$ awk '...' foo.fa
11  12  0.916667

If you need GC content per-FASTA element, the END block's code can be copied into the main loop:

$ awk ' \
BEGIN { \
    FS=""; \
    cg=0; \
    t=0; \
} \
{ \
    if ($1 != ">") { \
        for (i = 1; i <= NF; i++) { \
            if ($i ~ /[ACTGactg]/) { \
                t++;
            } \
            if ($i ~ /[CGcg]/) { \
                cg++;
            } \
        } \
    } \
    else { \
        if (t > 0) { \
            print h"\t"cg"\t"t"\t"(cg/t); \
            cg = 0; \
            t = 0; \
        } \
        h = substr($0,2); \
    } \
} \
END { \
    print h"\t"cg"\t"t"\t"(cg/t); \
}' foo.fa

The output from this includes each element's header:

$ awk '...' foo.fa
ctcf    11  12  0.916667

To get a FASTA file, if your organism-of-interest has reference data on the UCSC Genome Browser, you can obtain sequence data for your regions-of-interest through the use of a custom track.

First, create a BED file containing your regions-of-interest and upload that file to a custom track via My Data > Custom Tracks. Then ask the browser to return the sequence data associated with these custom regions:

1. Select Tools > Table Browser
2. Select Custom Tracks from the group pull-down menu
3. Select your custom track from the table menu
4. Set the region radio button to genome
5. Set the output format to sequence
6. Click get output
7. Set any post-processing options and click get sequence

Save the resulting FASTA file to a place where you can find it. Then run it through one of the two awk scripts above.

1. determine the slide window size
2. calculate the gc content for each window size

Please refer to http://www.biologicscorp.com/tools/GCContent which plots using google chart and it is easy to know how to calculate.