What have you already tried? What is the structure of your gene expression object? These details help us to make appropriate suggestions.

if you have a matrix of gene counts where the row names are genes and columns are samples, you can try the following:

# vector of genes of interest
genes <- c("Gene1", "Gene2", "Gene3")

# get vector of aggregated gene expression for each column in my gene count matrix
aggregated <- apply(gene_counts[genes,], 2, sum)